Tune Vale (and re-enable in CI if possible)

[neverett]

Summary & Motivation

Vale is a prose linter that we have configured to check md, mdx, and rst files in the docs directory. We used to run it as part of the docs CI, but it was noisy, so we removed it from CI, but not from the directory.

This PR updates Vale config to make it more useful and less noisy and annoying, and hopefully re-enable it in CI.

How I Tested These Changes

Ran Vale locally.

Changelog

Insert changelog entry or delete this section.