Create links to placeholder definitions in config

This will create local links of all placeholders in config blocks to
their respective definitions.

Add missed slack icon_url to AlertManager config.

Add consul and graphite exporters to downloads page

Several minor changes as discussed earlier

Community: Add section to discourage 1:1 support queries

I find myself quite often in the situation where I have to tell people
what I'm adding here. Even if not all will read it, it is then easy to
point people to it.

Forgot to add a link.

Add Nagios and Sensu, update existing ones.

Fix for clipped descenders in feature headings

Fix a spelling error.

This is per the notes from Promcon.
I decided to leave in the client library/exporter section,
but altered to be more hand wavy.

Also fix a few markdown style issues.

Document time range parameters to /api/v1/series

Fixes list formatting.

start and end parameters were added to the series API endpoint in #1625 (see #1542). This updates the online documentation to document this extremely useful feature

Next release

api: add docs for targets and alertmanagers api

Overview exporters: remove Ganglia, add Graphite