Update release-checklist and move into docs

[gvilitech]

Update the doxygen instructions in the release-checklist with these instructions:

• Create a virtualenv with pip3 install -r docs/requirements.txt and activate it.
• Run make html from the docs directory.

Move the release-checklist to be part of the docs.

[96RadhikaJadhav]

@gvilitech Can I try this one?

[gvilitech]

@96RadhikaJadhav Of course! :)