Reduce the number of redirected link errors

[TibsAtWork]

When make linkcheck is run, there are a lot of redirected links (search for redirect in the linkcheck output)

NOTE I'm going to split this up into smaller PRs, for ease of review...

This PR aims to fix those that are simple to fix:

• Using http instead of https when appropriate

• Replace developer.aiven.io with docs.aiven.io

  Note: not for _redirects.txt, as that's correctly saying developer.aiven.io redirects to docs.aiven.io

  I believe it is correct to alter robots.txt and conf.py

  We could come back later and turn some/many of these into :doc: references instead - see Use Sphinx :doc: roles for links to docs.aiven.io documents #1548

See #1485

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[TibsAtWork]

This PR has been replaced by smaller PRs.