Issue #26014: Guide 2.7 users to the new packaging documentation:

Issue #26014: Guide 2.7 users to the new packaging documentation:
- the top-level 2.7 docs page now links to the newer doc pages
  (installing and distribution) rather than the legacy ones
- the legacy install and distutils pages are now labeled as "legacy"
- the library/distutils page is updated to match the 3.x version
- "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)