DOC: remove older whatsnew
@jorisvandenbossche what do you think? should remove 0.17 as well? 0.18? (e.g. keep last 2 major releases).

Idea is that this will make docs a bit shorter / build faster (slightly). not sure if its is worth though. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15809?src=pr&el=h1) Report
> Merging [#15809](https://codecov.io/gh/pandas-dev/pandas/pull/15809?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/18ac0b7752b531daa105a73ef3c211f83bc5c8f7?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/15809/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/15809?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #15809      +/-   ##
==========================================
- Coverage      91%   90.99%   -0.02%     
==========================================
  Files         143      143              
  Lines       49384    49384              
==========================================
- Hits        44944    44936       -8     
- Misses       4440     4448       +8
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15809?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/15809?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/15809?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.56% <0%> (-0.1%)` | :arrow_down: |
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/15809?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.51% <0%> (ø)` | :arrow_up: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/15809?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.3% <0%> (+0.33%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15809?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/15809?src=pr&el=footer). Last update [18ac0b7...57510d3](https://codecov.io/gh/pandas-dev/pandas/pull/15809?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). I personally would not remove them. I am more in favor of https://github.com/pandas-dev/pandas/issues/6856 -> converting examples to static code-blocks (keeping behaviour as when released, instead of updating examples), which also makes it a lot faster to build.

Although I am maybe a bit too 'attached' to the older whatsnew docs (I regularly search through them, but maybe this is rather uncommon behaviour as I am a dev).  @jorisvandenbossche ahh I c, didn't even know that issue existed! ok that actually makes sense. I tagged that a bit. closing then. Something else we should do, is starting a separate page at a certain point, instead of one huge (or moving the older to a separate page, but problem with that is that it breaks links) @jorisvandenbossche yep, though I think we have a reorg on the whole doc structure somewhere....