Fixes some redirection links in docs.

- "See also" links to the new docs are now provided in the legacy pages
- links to setuptools documentation have been updated

(original patch by Susan Sun)

Closes #25910: fix dead and permanently redirected links in the docs. Thanks to SilentGhost for the patch.

- key term hyperlinks for distutils, setuptools & wheel
- distutils landing page links directly to setuptools docs
- short summary of setuptools benefits provided with link

Also applies to cherry pick Issue #20909.

- based on pip and other PyPA tools
- includes references to the new Python Packaging User Guide
  where appropriate (and the relevant section is at least
  partially filled in)
- started new FAQ sections
- both guides aim to introduce users to basic open source
  concepts if they aren't aware of them
- existing guides have been relocated (now linked from the
  distutils docs) rather then removed, since there is
  some needed material that has yet to be relocated to the
  distutils docs as a reference for the legacy formats