Pitch CloudNativeCon Prometheus videos on front page

It has unintentionally become a defacto listing of exporters, some of whom
we may not wish to list here for various reasons or for which noone has
sent a PR yet. This helps make it easier for users to find exporters,
and potentially aid in development of the ones that are still maturing.

FlowDock integration is not currently supported in .5 version of AM.

See https://github.com/prometheus/alertmanager/issues/236

Installation: mention Puppet module

The puppet prometheus module works fine with the current version of prometheus.

Remove remaining references to PromDash

* Document __meta_azure_machine_name
* Document __meta_gce_metadata_*
* Correct __meta_gce_tags_* (remove __meta_gce_instance_tags_*)
* Sort long lists alphabetically
* Link to relabeling section from all meta label lists.

It has been long enough. Sleep well, little prince.

This commit adds the ElasticSearch stats exporter

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.