Update BEAM docs

Improve beam docs:
 * correct spelling
 * update per pandoc changes
 * capitalize titles
 * capitalize BEAM throughout and use "the BEAM" when referring to the virtual machine.
 * tweak grammar and phrasing
 * reformat build-tools-rebar3 section
 * add more links
 * re-wrap <para>s

Also update <programlisting>s
* normalize whitespace
* don't double quote homepage
* use $ in all shell snippets