Reorganize information about the branches of the repo.

[ezio-melotti]

This PR achieves 3 things:

1. It tries to limit the number of information that needs to be updated regularly by removing links to the latest micro release;
2. it moves things that need to be updated (the list of active branches) to a single place (the index page);
3. it makes the index page simpler/shorter by moving some unimportant paragraphs to the devcycle page;

[willingc]

Looks great @ezio-melotti. Thanks for updating 👍

[jdufresne]

Could the section "For reference, here are the most recently end-of-lifed release cycles:" along with the table of versions be restored?

FWIW, I reference this table very frequently when discussing dropping EOL Python support in projects. I went to reference it this morning and had trouble finding it. If this isn't the appropriate page, would some other page work?