Resolve latest linkcheck issues #473

[phillxnet]

Addresses some outdated, and robot sensitive links via workarounds / removals etc. N.B. more specific content improvement is needed for such things as our now legacy Shop to Open Collective changes, i.e new pics, but this issue focuses on restoring a healthy link state. Not a full re-write in all cases where modifications were required.

Noteworthy major changes:

• Shop replaced by Open Collective - rough first pass.
• FAQ repo overview updated.
• Remaining btrfs wiki links replaced by readthedocs.
• Removed owncloud.rst as no longer a Rock-on.
• Recorded deletion of owncloud.rst in redirects.txt

Fixes #473

Checklist

• [x] With the proposed changes no Sphinx errors or warnings are generated.
• [x] I have added my name to the AUTHORS file, if required (descending alphabetical order).

[phillxnet]

I have duplicate the GitHub actions checks locally, using this PR, with the same results. As we now have a return to health link-wise, doc wide, I'll merge and publish this PR's product as it fixed what it set-out to fix.

[phillxnet]

We have an a members/membership typo but not worth delaying this PR, plus we have the follow-up in that area re image refresh etc. We also still reference Rockstor 4 when we are now at 5 + but again these are not broken links / or old redirects.

[phillxnet]

PR product PRODUCTION published.