Add development documentation on label filtering

Closes #49651

See merge request gitlab-org/gitlab-ce!30741

Add points how to prepare for database reviews

Closes #65126

See merge request gitlab-org/gitlab-ce!31158

Following the single docs codebase change, all internal links
should be relative. Also cleans up one table

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/65126.

Came up in the database office hours call on 2019-07-25.

We have a fairly good guide to Code Reviews, but can be improved
by adding a few examples of what a good code review looks like
at GitLab, specifically ones where there is a bit of back and
forth, "nit-picking," etc. This would:

+ help set expectations of newly hired engineers around what our
process looks like when it is functioning what level of scrutiny
their code will be under

+ how we have technical conversations

+ show by example how after you're done crafting a solution, there
can still be extra work done either tidying up code and/or managing
the communication and conversations about your proposed MR

This topic comes up periodically, and we've investigated several options
for changing the way we filter labels. Unfortunately, we have yet to
find an option that is strictly better than the current state.

Add documentation on how to add dependencies to yarn

See merge request gitlab-org/gitlab-ce!31066

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