Fix links broken by move to SciTools, and update RTD link.

[pp-mo]

Closes #198

Note that:

• can't immediaately see effect, as we don't do linkcheck on PRs
  (because it isn't actually that simple)
• so, hopefully, will pass a manual invocation, once merge
• won't pass tests, due to Xr202601 identical fix, take 2 #197 problem outstanding