Many code blocks are 4spaced, and they render in GitLab
without coloring as a result, even though they are
fenced with a language label. If in a list, other items
will render as being in a code block too, even if not
meant to. This fixes all these issues for many admin
docs in /high_availability and /auth (part 1)

The consul docs are currently in the HA database section, but it should
be in it's own section.

The installation link has changed from /installation/ to /install/. This
commit will update all the remaining links that are missing.

Also fixes instances in Markdown that broke the rule.

Previously, we used brackets to denote the tier badges,
but this made Kramdown, the docs site Markdown renderer,
show many warnings when building the site. This is now
fixed by using parentheses instead of square brackets.

This was caused by [PREMIUM] looking like a link to
Kramdown, which couldn't find a URL there.

See:
- https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/484
- https://gitlab.com/gitlab-org/gitlab-ce/issues/63800

Also adds and one linting rule
and makes project conform to it.

Updating documentation to include
Prometheus Service Discovery. Added
monitoring node documentaiton as
well as documentation for the
individual service nodes.

Add instructions for enabling the `pg_trgm` extension in the production db. As a workaround to [a fault during HA setup](https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2501).

In a high availability configuration, the Redis and PostgreSQL nodes
should not attempt to run database migrations. In fact, trying will
result in errors about not providing a database password. To
prevent errors and confusion, add this configurtion to these nodes'
`gitlab.rb` file.

postgresql['sql_password'] is not used, should be
gitlab_rails['db_password']