PR: Fix Jinja/MarkupSafe pinning, upgrade hooks and swap rstcheck for sphinx-lint

[CAM-Gerlach]

Pull Request

Pull Request Checklist

• [x] Read and followed this repo's Contributing Guidelines
• [x] Based your PR on the latest version of the correct branch (master or 4.x)
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• [x] Described your changes and the motivation for them below
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Description of Changes

Sphinx 3 uses various constructs deprecated in Jinja 3.0 and removed in 3.1, as well as MarkupSafe 2.0, but does not properly upper-cap them to supported major versions. Since we're still stuck on Sphinx 3 for now, I've pinned us to Jinja <3 (as I should have well before) to get things working again.

I've also done a routine update of the pre-commit hooks, and swapped out the fragile, expensive and somewhat broken rstcheck for the more useful and lightweight sphinx-lint, fixing a couple errors it spotted along the way.

[CAM-Gerlach]

Since this is mostly just an emergency fix to the pinning to unbreak the builds plus routine hook updates, and this is blocking @jitseniesen 's #314 to fix #313 , and I'll need to do a tricky merge to master to resync them, I'll just merge it now since everything's passing again and it doesn't touch the actual build system/content.