Docs Quick tip: share slugify filter with Markdown TOC

[zachleat]

First noticed it on here but it’s a good idea: https://twitter.com/codingsafari/status/1410682849473277961

Highly related to 11ty/eleventy#278 and the new slugify replacement.

[pdehaan]

That's a neat trick!

Bonus points if we can use the eleventyConfig.getFilter("slug") API instead of eleventyConfig.javascriptFunctions.slug.