Expand Pushgateway docs with Java example.

Be consistent in saying 'Pushgateway'.

Copy template docs over from the wiki.

Add links to client code for the pushgateway.

Make getting started link on frontpage go to Getting started section.

Put exposition formats at the end of instrumenting, as it's the least relevant
to normal users.

fix a broken link

Followup fixups for PR #2.

This introduces a lot of small followup changes to
https://github.com/prometheus/docs/pull/2.

Some things are clear right/wrong spelling/grammar issues, while others
are a matter of style or taste.

Except in extreme cases, I didn't reflow the text aggressively to a
certain number of columns, in order to make it easier to spot
differences.

Let me know what you think.

Some reference links worth mentioning:

Compound adjectives:
http://www.grammar-monster.com/lessons/adjectives_compound_adjectives.htm

Run-on sentences:
http://grammar.ccc.commnet.edu/grammar/runons.htm

General tech-writing style guide (which I haven't actually looked at
much yet):
http://infobits.eu/tech-doc/style-guide/

Add lots of docs, mostly best practices.

Start out on the visualisation docs.

Expand overview, remove federation as a feature we don't have that yet.

Switch example variable to follow naming scheme.

Add Dockerfile