Clarify current exposition format support

Fixes prometheus/alertmanager#1221

PingCAP is the team behind [TiDB](https://github.com/pingcap/tidb), which is using  Prometheus to monitor the database.

It has been changed to use the Pagerduty v2 API by default in Alertmanager.

https://github.com/prometheus/alertmanager/blob/master/config/config.go#L324

Add governance

Add release 2.1 of Prometheus

Signed-off-by: Goutham Veeramachaneni <cs14btech11014@iith.ac.in>

[This pull-request](https://github.com/prometheus/prometheus/pull/984) renamed the `__meta_dns_srv_name` to `__meta_dns_name`.

Mostly language and style fixes. Cross-project discussion _should_
happen on the mailing list.

and instead make a new "page" layout for markdown-based pages. This
avoids changing the HTML based pages which are using the "default"
layout currently.

Let the governance use the default layout (all markdown files default to
"docs").

Make the default layout use the right div classes so the CSS can take
hold.

Fix the `layout` declaration for Markdown documents that are not `kind:
article`. Declare the governance to use the "default" layout, not the
"docs" one. This is the minimal change to support the governance
document. The layout choice needs a refactoring, but how to do that
needs more thought.

and add the nanoc annotation.

as discussed in the team call, this does not need to be split onto the
community page if the governance itself is on prometheus.io.

as proposed there, this should be a documentation page.

Gnocchi can be used as remote endpoints and storage.

This change adds this in the documentation.

* Fixed spelling mistake

* Removed extra line

Quick pass on the writing exporters docs to make them clearer and more consistent