Docs: Improve the release notes index page

Moved the upgrade notices from changelog to release notes index, included
a link back to changelog from release notes index, and changed to
link to TTK release notes instead of changelog.
Docs: Move 2.1.1 changelog to release notes

This way all the releases that can be directly upgraded to latest release
now have release notes, that are much more detailed than previous changelog.

The release notes were retrieved from SourceForge.
Docs: Add more detailed upgrade instructions
Docs: Remove unnecessary upgrading instructions
Include the assets rebuilding in setup management command

This saves us a step in installation/upgrade docs.