gh-140715: Improve class reference links on datetime.rst

[uatach]

Another pull request based on PR #21447, this one is focused on improving class reference links.

• Issue: Wrong/missing code formats in datetime documentation #85453

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📚 Documentation preview 📚: https://cpython-previews--123980.org.readthedocs.build/

• Issue: Update str*time to C11 format codes #140715

[erlend-aasland]

I compared the rendered preview with the existing docs, and there are still quite a few places where I think the link density is too high within a paragraph. I'll jot them down tomorrow or the day after.

[uatach]

I compared the rendered preview with the existing docs, and there are still quite a few places where I think the link density is too high within a paragraph. I'll jot them down tomorrow or the day after.

@erlend-aasland did you have a chance to look into this?

[hugovk]

Thanks for this PR, this is an improvement. If there's more suggestions, we can do that in a followup. But let's merge this as it's been open a while.

Whilst we're here, I'm going to push an update to put the headers in sentence case (see style guide) and will merge tomorrow if no objections.