I noticed the doc/development/testing_guide/best_practices.md still
referenced the `[run mysql]` tags, etc. They no longer work, so I
removed them, then realised I had better clean up the rest of
doc/development !
Some markdown headers needed tweaking to adhere
to standards, including blank lines above and below,
only one space after hash, first header should be
h1, and only one h1 per doc
In GitLab EE, a GitLab instance can be read-only (e.g. when it's a Geo
secondary node). But in GitLab CE it also might be useful to have the
"read-only" idea around. So port it back to GitLab CE.
Also having the principle of read-only in GitLab CE would hopefully
lead to less errors introduced, doing write operations when there
aren't allowed for read-only calls.
Closesgitlab-org/gitlab-ce#37534.
