Support common snippets in content repositories

[smashwilson]

Different content repositories often share common sections of text. Keeping these sections up to date and consistent across all of the content repositories is error-prone and a chore. Just as we refactor common source code into reusable modules, writers should be able to refactor common docs into a shared, centralized place.

Most content engines have some capability for "including" common content within the repository, but they generally don't have an external include. To solve this, I'd propose writing Sphinx and Jekyll plugins that fetch and inject raw content from Somewhere ™️. We could host them in the content service (as a separate resource, to keep them from mucking up search) but honestly I'd probably just pull directly from raw.githubusercontent.com in some repo.

/cc @meker12