Use Sphinx RTD Theme
- [X] closes #15556 

TOC on the left hand side is pinned during navigation and also expandable/collapsible

<img width="1089" alt="image" src="https://user-images.githubusercontent.com/609873/54065608-3cc81e00-41d8-11e9-830d-124d759bf791.png">


<img width="1099" alt="image" src="https://user-images.githubusercontent.com/609873/54065614-62552780-41d8-11e9-8edc-3bd7db452ca1.png">


# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25614?src=pr&el=h1) Report
> Merging [#25614](https://codecov.io/gh/pandas-dev/pandas/pull/25614?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cc4a7e510f4bbd5aeb8138de16b888f4ce1551da?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25614/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25614?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25614      +/-   ##
==========================================
- Coverage   91.26%   91.26%   -0.01%     
==========================================
  Files         173      173              
  Lines       52968    52968              
==========================================
- Hits        48340    48339       -1     
- Misses       4628     4629       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.83% <ø> (ø)` | :arrow_up: |
| #single | `41.71% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25614?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25614/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.57% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25614?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25614?src=pr&el=footer). Last update [cc4a7e5...fe39855](https://codecov.io/gh/pandas-dev/pandas/pull/25614?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25614?src=pr&el=h1) Report
> Merging [#25614](https://codecov.io/gh/pandas-dev/pandas/pull/25614?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cc4a7e510f4bbd5aeb8138de16b888f4ce1551da?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25614/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25614?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25614      +/-   ##
==========================================
- Coverage   91.26%   91.26%   -0.01%     
==========================================
  Files         173      173              
  Lines       52968    52968              
==========================================
- Hits        48340    48339       -1     
- Misses       4628     4629       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.83% <ø> (ø)` | :arrow_up: |
| #single | `41.71% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25614?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25614/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.57% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25614?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25614?src=pr&el=footer). Last update [cc4a7e5...a97bb23](https://codecov.io/gh/pandas-dev/pandas/pull/25614?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Should we be re-styling the website at the same time as the docs? Or would people be OK with changing those at different times?

@WillAyd have you looked at the http://pandas-docs.github.io/pandas-docs-travis/user_guide/style.html page with this theme? Does it look OK? @TomAugspurger sorry just seeing this now. That theme looks OK to me!

I'm not terribly tied to the this PR but opened because:

  1. I think it's regardless an improvement over the nature theme
  2. Would be entirely external to pandas (no theme vendoring or changes required)
  3. Has a better dropdown on the left (i.e. doesn't require navigating to page to expand options)
  4. It's easy

If there's not an appetite for it though certainly can close > Should we be re-styling the website at the same time as the docs? Or would people be OK with changing those at different times?

To answer this explicitly I think different times is fine. I don't see a super tight harmony between existing site and docs (though more than RTD) and wouldn't want to try a super big change at once. That's fair.

Thoughts on holding off on this till we hear if we line up someone to work on our docs as part of that NUMFocus grant? Yep works for me - no real urgency here. 

Sent from my iPhone

> On Mar 14, 2019, at 2:26 PM, Tom Augspurger <notifications@github.com> wrote:
> 
> That's fair.
> 
> Thoughts on holding off on this till we hear if we line up someone to work on our docs as part of that NUMFocus grant?
> 
> —
> You are receiving this because you were mentioned.
> Reply to this email directly, view it on GitHub, or mute the thread.
