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
The documentation got a bit stale compared to actual contents of
nixpkgs. This commit focuses on updating existing docs, not on making
sure all details of beam packages are covered.
