Use Python-Markdown 3.3.7 during site deployment

[ericseppanen]

We are occasionally bitten by a bug in Python-Markdown <= 3.3.6 that causes lines like this:

* [add `[T]::as_simd(_mut)`](https://github.com/rust-lang/rust/pull/91479)

to be mis-parsed, resulting in a blank line being emitted.

I see that Python-Markdown 3.3.7 has been released, which hopefully has the bugfix. So hopefully we can stop seeing this bug if we can:

1. Force use of Python-Markdown 3.3.7 when deploying the site (@nellshamrell, is this possible?)
2. Force use of Python-Markdown 3.3.7 in CI
3. (optional) remove the workarounds (pointless   on the end of the troublesome lines)

See #2781, #3336, ade6ded00bf0 for places this bit us in the past.