Add Releases to readme and add all conformance classes to build index

[philvarner]

Related Issue(s): n/a

Proposed Changes:

1. Add Releases list to Readme to make it more clear where to find specific versions of STAC when we move to only having a main branch
2. Add all the conformance classes to the build index

PR Checklist:

• This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• This PR has no breaking changes.
• This PR does not make any changes to the core spec in the stac-spec directory (these are included as a subtree and should be updated directly in radiantearth/stac-spec)
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.