Migrate documentation to versionless "read the docs" DNS slug

[carl-adams-planet]

Ticket to track our progress on moving docs from planet-sdk-for-python-v2.readthedocs.io/en/latest/ planet-sdk-for-python-v2.readthedocs.io

Requirements:

• Update the builds to publish to new site
• Update Planet corporate docs to point to the updated site. This includes all docs under the following:
  • docs.planet.com
  • Other sites TBD
• Switch the "default" documentation version to a 3.0 version once 3.0 has achieved stable release status
• Retire old site with a redirect.