DOC: Add new required sphinx setting latex_elements
nbsphinx released a new version,  `0.4.0`, and it requires a sphinx setting that we don't have.

This is making the doc build fail, this PR adds it so the doc build works again.

CC: @jreback 

Hello @datapythonista! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`doc/source/conf.py`](https://github.com/datapythonista/pandas/blob/139959390b5f7d7dfdcd1601cab1d20d24d4092f/doc/source/conf.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24291?src=pr&el=h1) Report
> Merging [#24291](https://codecov.io/gh/pandas-dev/pandas/pull/24291?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7b0fa8e94e22aa4a491fa574bc56f8ca791f7608?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24291/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24291?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24291   +/-   ##
=======================================
  Coverage   92.22%   92.22%           
=======================================
  Files         162      162           
  Lines       51824    51824           
=======================================
  Hits        47795    47795           
  Misses       4029     4029
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.62% <ø> (ø)` | :arrow_up: |
| #single | `43.01% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24291?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/24291?src=pr&el=footer). Last update [7b0fa8e...1399593](https://codecov.io/gh/pandas-dev/pandas/pull/24291?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/24291?src=pr&el=h1) Report
> Merging [#24291](https://codecov.io/gh/pandas-dev/pandas/pull/24291?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7b0fa8e94e22aa4a491fa574bc56f8ca791f7608?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24291/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24291?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24291   +/-   ##
=======================================
  Coverage   92.22%   92.22%           
=======================================
  Files         162      162           
  Lines       51824    51824           
=======================================
  Hits        47795    47795           
  Misses       4029     4029
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.62% <ø> (ø)` | :arrow_up: |
| #single | `43.01% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24291?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/24291?src=pr&el=footer). Last update [7b0fa8e...1399593](https://codecov.io/gh/pandas-dev/pandas/pull/24291?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 maybe better to just pin the version until we know ifbit works ok? As you prefer, but I think it was just that our `conf.py` was generated so much time ago, that `nbsphinx` was assuming every project would have a setting that sphinx added enough time ago.

I think it's just a normal update, nothing buggy on it. In any case, won't be able to create the PR for pinning the version myself until Sunday probably. If that's preferred, someone else should do it. thanks!

have a look at built docs when they r done. The docs look good. I think the only page that could be affected by the `nbsphinx` update, is this one as it's the only `.ipynb`: https://pandas-docs.github.io/pandas-docs-travis/style.html

And it looks good. Also checked the `ipython` directive rendering in other pages, and nothing seems to be changed.