Add types-channels installation docs and extras

[huynguyengl99]

Hi @carltongibson and @collinanderson, after some work, types-channels has been completed and pushed to PyPI at https://pypi.org/project/types-channels/ (working PR: python/typeshed#13939). To help users discover and use the type package, I have updated the channels documentation. Can you take a look at the PR #2170 and give me some feedback or approve it? Thank you.

Changes

• Add types extra to setup.cfg
• Add type checking installation docs to installation.rst
• Add types-channels reference to community.rst