Fix broken links, set linkcheck_rate_limit_timeout=10

[stevepiercy]

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[stevepiercy]

@ksuess this is the first of three PRs to bring the build stack up to date for Training.

There is a new configuration for linkcheck_rate_limit_timeout = 10.0 which is ignored as an int so needs to be set as a float for Sphinx<=7.3.7. Even then, it stalls on https://github.com/search, so I just ignored that URL. Anyway, now linkcheck is back to taking about 5 minutes to complete, instead of over 90 minutes.