Support partial includes

Problem to solve

When writing documentation, especially when following a runbook-style, some parts of the documentation may be duplicated across individual pages of documentations. This is not very DRY and essentially means there is no single source of truth.

Solution

Support partials for markdown files with shared documentation; for example, short snippets for reconfiguring GitLab. This is supported in other tools: https://gohugo.io/content-management/shortcodes/

Here is an example of where I'd like to use partials: gitlab-org&4266

cc @cnorris