DOC: Add pint pandas ecosystem docs
- [ ] closes #xxxx (N/A)
- [ ] tests added / passed (N/A)
- [ ] passes `black pandas` (N/A)
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff` (N/A)
- [ ] whatsnew entry

@datapythonista thanks for your help with this (and #22582). What did you mean when you said the docs had moved? Didn't realize we still have the ecosystem page in the docs. We moved it to the [web](https://github.com/pandas-dev/pandas/blob/master/web/pandas/community/ecosystem.md), but I think we finally didn't have an agreement on which version to keep. Do you mind duplicating the changes in the web file here, so they keep synchronized. Ok done (also added pandas path into the list of accessors for completeness) Great thanks @znicholls 