Update extensions readme

[m-mohr]

Related Issue(s): #1203

Proposed Changes:

1. Remove the list of STAC extensions, only refer to stac-extensions.github.io for easier maintenance and updates
2. A lot of changes and improvements in wording/text, cater for recent changes e.g. in how we handle and write STAC extensions (scope list)
3. Better explain that you can basically extend every object
4. Tried to remove a bit of duplication
5. Moved extension maturity a level up as it is important for most chapters, not only for extending STAC

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.
• I have added my changes to the CHANGELOG
  or a CHANGELOG entry is not required.
• This PR affects the STAC API spec,
  and I have opened issue/PR #XXX to track the change.

[cholmes]

looks good, thanks!