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: https://gitlab.com/groups/gitlab-org/-/epics/4266
cc @cnorris
issue