This commit tweaks the language in Analytics.md to make it clear
that summaries of install/error data are publicly available,
while only Homebrew maintainers can access the full analytics
(as specified above in the document).

We didn't end up using the `screenview` and `exception` analytics as
much as expected so let's remove them and focus on stuff that's
formula-specific.

Also changes some heading levels so only one primary
heading appears per page.

Unfortunately there's no easy way to share this code with the brew.sh
site but it's at least able to have the `css/`, `img/` (root) and
`_layouts` directories in sync between the two repositories.

Some small CSS tweaks were needed to make these pages render more
attractively.

GitHub now nicely generates a documentation site for us at
http://brew.sh/brew based on our docs folder. Optimise the output of
this and the GitHub docs directory for readability and the various user
groupings.