chore: improve Material for MkDocs config

[HonkingGoose]

Changes:

• Configure automatic light / dark mode [^1]
• Watch the includes directory, so the preview updates when you edit the abbreviations.md file

Context:

Material for MkDocs reached another funding goal [^3], so we get access to new features. :partying_face:

There's a privacy plugin [^2], but that only works with fully qualified URLs, and we autogenerate a lot of stuff with scripts. So I don't know if the Privacy Plugin would work properly.

[^1]: Material for MkDocs, automatic light / dark mode [^2]: Material for MkDocs, privacy plugin [^3]: Material for MkDocs, $14,000 Goat's Horn features

[viceice]

you can test the privacy plugin 😉