Add 8.x branches for Kibana

[lcawl]

This PR updates the conf.yaml to start building 8.16 Kibana Guide and Installation and Upgrade Guide from the 8.x branches.

It also:

• Removes attributes that were added to work around broken links from other books, since the full set will now be live
• Adds a conditional clause to the code that checks the Kibana UI links, since otherwise it was seeking an 8.16 branch
• Cleans up a "main" branch from one of the conf.yaml lines related to the v6.2 shared attributes, since that branch doesn't care about those attributes.

[github-actions]

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[lcawl]

run docs-build

[bmorelli25]

Super neat!