DOC: ecosystem pandas-stubs information
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

Added information about pandas-stubs to both the .rst and .md ecosystem doc files.

closes https://github.com/pandas-dev/pandas/issues/43197
@datapythonista 

Decided to make some changes based on your feedback:

* Reduced the amount of technical detail
* Included an installation guide
* Included an example @datapythonista Do I have to take some action for the CI to pass? It times-out after an hour. /azp run <samp>
Azure Pipelines successfully started running 1 pipeline(s).<br>

</samp> > @datapythonista Do I have to take some action for the CI to pass? It times-out after an hour.

I have just restarted CI now. Hopefully should run to completion this time pre-commit raises errors. You should fix them first I will get around to fixing the pre-commit issues locally. thanks @zkrolikowski-vl 