chore(docs): add label to version selector in site header

[wphillipmoore]

The version selector in the MkDocs Material site header shows the version (e.g., "dev") without any label, making it easy to overlook.

Add the same CSS override applied in vergil-project/vergil-tooling#886:

1. Create docs/site/docs/stylesheets/extra.css with a ::before label on .md-version
2. Add extra_css: [stylesheets/extra.css] to docs/site/mkdocs.yml

Reference implementation: vergil-project/vergil-tooling#920

[wphillipmoore]

Fixed in #145