Develop Category re-org and updates to V3 docs

This pull request contains:

Updates to V3 docs for clarity
A change in the structure of the "Develop" section of the docs to be more user friendly. Different categories of products now get their own section instead of all being lumped into V2.

Work still needed:

[ ] Finish updating V3 docs edits
[ ] Confirm accuracy of existing documents being migrated to new places
[ ] Add smart contracts section to v3 docs. This requires building the documentation from natspec in the contracts. see this issue: https://github.com/yearn/yearn-devdocs/issues/430
[ ] update testing section in Building on Yearn. Confirm other parts of this page are accurate as it was modified from an older hackathon document (hacking on yearn)

yearn / yearn-devdocs