Menu item to auto-slugify internal links

Converting internal section links to slugs in Markdown is a recurring pain. And very confusing for certain sections with numbers or punctuation because nuances of slugging rules (see [github-slugger](https://github.com/Flet/github-slugger)).

- Instead of writing `[some section](#some-section-blah-blah)` let people copy/paste the section heading as `[some section](#Some Section Blah-Blah)`
- Have a menu item that runs github-slugify on all internal section links that have an uppercase letter or a space in them. (Or it may be safe to slugify all.)
- This would have no effect on all valid slugs.

Bonus features:
- Another option to count all internal links that don't fit the TOC, check for them, and add `badlink.` to any that are not present in the doc.
- Note this wouldn't handle duplicate sections. Could add a warning message about that.