Introduce categories to exporterlist

As @juliusv mentioned once in #prometheus, the list of external exporters has grown quite a bit. This attempts to increase overview by introducing categories for the third party exporters.

The chosen categories are a first proposal and might still be subject to change, depending on the feedback, any of which is greatly appreciated!

This reverts commit c0f332ee.

Upon request by Puck` on IRC.

Document alert_relabel_configs.

Fixes #568

Add base units requirement to naming best practices doc

Add prometheus-users@ to Community page

Fix typos across the docs repo.

Fixed some blog and documentation typos.

Add Incopro interview

Added ProxySQL exporter to the list.

Add information on using pre-built / contributed dashboards.

Fix `label_replace` documentation

Updated (typos and copy clarity etc.) following PR comments.

With the fix in place, the documentation is in line with the intention
expressed by tests,
e.g. https://github.com/prometheus/prometheus/blob/master/promql/testdata/functions.test#L173-L176
and
https://github.com/prometheus/prometheus/blob/master/promql/testdata/functions.test#L195-L199
.

querying examples: fix proctype vs proc metric key

Add Compose interview

Add Cloud Foundry Firehose Exporter to documentation

Add Prometheus-Talk @ MRMCD 2016

Change vector.im -> riot.im on Community page

Vector is Riot now, see:

https://techcrunch.com/2016/09/19/riot-wants-to-be-like-slack-but-with-the-flexibility-of-an-underlying-open-source-platform/

Added GitHub Exporter to documentation

Remove examples of time metrics not using seconds

Bring visualization template example up-to-date

Add interview with Digital Ocean