Linkcheck cleanup (#490)

[Hooverdan96]

Fixes #490.

This pull request's proposal

Link cleanup/replacement where applicable, based on link checkup

Checklist

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

[phillxnet]

@Hooverdan96 Thanks again for your continued maintenance re our plethora of links. I think we should consider reducing our link density as it goes, i.e. as per the recently merged nginx Rock-on write up. We only really need to give a pointer of two to upstream projects, and then only to their canonical references: i.e. no need to also link in companies etc. We are not a link farm after all :) .

This branch looks to do what it said on the tin so I'll merge and get published shortly.

[phillxnet]

Local build test

This branch only.

rediraffe: Redirect generation skipped for linkcheck builders.
build succeeded.
Look for any errors in the above output or in _build/linkcheck/output.txt
Link check complete; look for any errors in the above output  or in _build/linkcheck/output.txt.
program finished with exit code 0
elapsedTime=27.830662

[phillxnet]

PR product PRODUCTION published.