doc: Update out-of-date links

Some external projects have bumped their versions or changed the
addresses of their documentation.
Update the URLs of the related links in Kronos docs.

- Update links to Critical App Monitoring latest documentation.
- Update the link to "scarthgap-dev" branch of Cassini in README.
- Switch TF-M documentation links to TF-M RTD. Also put notes to
  highlight that Kronos docs and code specify different versions
  of TF-M.
- Other minor link updates/fixes.

Signed-off-by: David Hu <david.hu2@arm.com>