First batch of the overhaul of the add-ons section

[sneridagh]

Ok, this is the beginning only. I plan to continue tomorrow.
I cleaned up the big bucket that the main index.md was.
Not sure of its current shape yet but organizing this is daunting.
I merged some things that were already reordered in the training, merged all together in here.
I have the feeling that we will need a brother section in development section only for developing add-ons with all the workflow explained in there (testing, linting, etc).

Disclaimer: I haven't double check things yet!

• I signed and returned the Plone Contributor Agreement, and received and accepted an invitation to join a team in the Plone GitHub organization.
• I verified there aren't other open pull requests for the same change.
• I followed the guidelines in Contributing to Volto.
• I succesfully ran code linting checks on my changes locally.
• I succesfully ran unit tests on my changes locally.
• I succesfully ran acceptance tests on my changes locally.
• If needed, I added new tests for my changes.
• If needed, I added documentation for my changes, either in the Storybook or narrative documentation.
• I included a change log entry in my commits.

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