DOC: replace developer meeting with contributor community page
Updating the developer meeting page to include the new contributor meeting and slack
Along the way, I decided to include other communication channels and did my best to describe them, making this more of an internal contributor community page. 

Inspired by [SciPy](https://scipy.org/community/) and [NumPy](https://numpy.org/community/)'s community pages, and Matplotlib's [home page resources](https://matplotlib.org/) and [meeting documentation](https://matplotlib.org/stable/devel/contributing.html#get-connected), and [readme](https://github.com/matplotlib/matplotlib#contact). 

I tried to keep this pandas' community page as much as possible. Please provide feedback on what you would like to see changed, so it's more appropriate.

I would suggest we add redirects for the old page to this one. If you have an example for how this was done before, I could add it to this PR. Otherwise, I will investigate if Sphinx supports this.
> I would suggest we add redirects for the old page to this one. If you have an example for how this was done before, I could add it to this PR. Otherwise, I will investigate if Sphinx supports this.

We currently have some manual setup for redirects in conf.py, but tailored to generated API pages. Either you could adapt that a bit to work with generic pages, or otherwise we can use some sphinx extension (eg https://documatt.gitlab.io/sphinx-reredirects/index.html) I think I just added the redirect – but check me 😉 Looks good, labelling 1.5.1 for now so that we can backport if there aren't any objections from others The doc build failure seems to be related:

```
 /home/runner/work/pandas/pandas/doc/source/development/index.rst:26: WARNING: toctree contains reference to nonexisting document 'development/meeting'
looking for now-outdated files... none found
pickling environment... done
/home/runner/work/pandas/pandas/doc/source/development/community.rst: WARNING: document isn't included in any toctree
```

The `/docs/source/development/index.rst` file still needs to be updated > Looks good, labelling 1.5.1 for now so that we can backport if there aren't any objections from others

Generally we don't do the effort of backporting doc fixes, but given that it would be useful to have this updated soon and this shouldn't give conflicts, sounds good to me.