DOC: Add link to previous versions documentation
Add a "Previous versions" block with link to https://pandas.pydata.org where links to the documentation of prior versions are available. See preview below.

![image](https://user-images.githubusercontent.com/22901938/135364568-9f22f930-b7b4-42c7-ba39-c57f6290afc2.png)

- [x] closes #43782 
- [ ] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [ ] whatsnew entry

looks reasonable.

@pandas-dev/pandas-core if any comments. thanks @DrGFreeman  @meeseeksdev backport 1.3.x Something went wrong ... Please have  a look at my logs. BTW, for a longer term / better solution: there is active work in the upstream theme to add a version switcher (https://github.com/pydata/pydata-sphinx-theme/pull/436). So that will make it more accessible to get to the older versions.