SEO the docsite: handbook and docs

[aileenrose]

1. Add frontmatter metadata to all pages - at the very least
   description:
   canonical_path:
   twitter_image:

2. Generate a sitemap file for all of the published pages i.e.
   for handbook: about.sourcegraph.com/sitemap-handbook.xml
   for docs: docs.sourcegraph.com/sitemap.xml