DOC: Adding version to the whatsnew section in the home page
@TomAugspurger @jorisvandenbossche 

Not so important, but looking at the home, feels quite weird to have the "What's New" section first thing in the toctree, without specifying what's new of in which version.

May be it's just me, feel free to dismiss this, but wanted to open as it's a trivial change, and IMO makes it much clearer.

You can see how this looks here: https://datapythonista.github.io/pandas-doc-preview/

Sorry for the last minute stuff.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24929?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@0bc2d3e`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24929/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24929?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #24929   +/-   ##
=========================================
  Coverage          ?   92.38%           
=========================================
  Files             ?      166           
  Lines             ?    52389           
  Branches          ?        0           
=========================================
  Hits              ?    48398           
  Misses            ?     3991           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (?)` | |
| #single | `42.89% <ø> (?)` | |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24929?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/24929?src=pr&el=footer). Last update [0bc2d3e...d492a80](https://codecov.io/gh/pandas-dev/pandas/pull/24929?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/24929?src=pr&el=h1) Report
> Merging [#24929](https://codecov.io/gh/pandas-dev/pandas/pull/24929?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0bc2d3e0bf81019ece6003113e8e2ef0e24382b4?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/24929/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24929?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24929      +/-   ##
==========================================
- Coverage   92.38%   92.38%   -0.01%     
==========================================
  Files         166      166              
  Lines       52388    52389       +1     
==========================================
  Hits        48397    48397              
- Misses       3991     3992       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (ø)` | :arrow_up: |
| #single | `42.9% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24929?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24929/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.04% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/24929/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXBpLnB5) | `100% <0%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24929?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/24929?src=pr&el=footer). Last update [0bc2d3e...6043238](https://codecov.io/gh/pandas-dev/pandas/pull/24929?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I think I'd prefer `What’s New in 0.24.0` to `What’s New in 0.24.0 (January XX, 2019)`. Thoughts on that? Long term, it is indeed easy to forget to update that. Is there a sphinx substitution that could be used?