Delete trailing whitespace, fix blockquotes, fix note
boxes, with indentation, clean up tables, etc.

Add BrowserStack details to Frontend Dev Guide

See merge request gitlab-org/gitlab-ce!31744

Adds MD010 (Hard tabs), MD012 (blank lines), MD029 (ordered
list prefix), MD030 (spaces after list markers), and fixes
remaining docs that were failing these tests

Fix links as discovered by the lint in the docs project

This will ultimately replace the stats.js that
exists in EE.

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 most docs in
/development, and cleans up other whitespace issues too

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

Ensure that all numbered lists use only 1. and no other numbers.
Also ensure that numbered lists use proper spacing.