fix: Make the Documentation compatible with Sphinx v 5

[Mariatta]

Updated conf.py and specify the language.

Sphinx v5 was just released, and lang = None no longer works.

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Fixes #<issue_number_goes_here> 🦕

[Mariatta]

Not needed on the main branch since it's pinned to Sphinx 4.

[dandhlee]

Hey Mariatta, if there are any reasons to update Sphinx to version 5 I'll update the client library maintainers, but I haven't had the chance to fully explore if this is required yet. Please reach out to me if you'd like to use Sphinx 5 though 😃

[Mariatta]

@dandhlee There's an optimization in Sphinx v.5.0.0b1 that makes building the docs (sphinx-build) faster.
This particular change: sphinx-doc/sphinx#10337

[dandhlee]

:O Let me test this out and see if this is true; aiplatform docs take roughly 10 minutes to build, this would be an incredible feat.