Add consul node metadata documentation

Add k8s ingress discovery

	Marathon SD: Change port 'ordinal' to 'index' 

...to reflect the name change in:
https://github.com/prometheus/prometheus/pull/2865/commits/ab4d64959f117ca64ba9c099b60965fb55231968

This label was added in:
https://github.com/prometheus/prometheus/pull/2865/commits/d7f8e227a80b0ea3f652f51d4a5e1afa3aeb092e

Added OpenStack service discovery docs.

Update storage documentation for the Prometheus 1.6 release

Add remote_read docs and update remote_write ones

Next step: add example code for remote read, point more precisely to
examples and specs for each.

Document custom query timeouts.

Next release

api: add docs for targets and alertmanagers api

Overview exporters: remove Ganglia, add Graphite

There is no Ganglia exporter listed in the [exporters doc](../instrumenting/exporters.md) and support for gmond has been deprecated from the Node exporter, so I think we should remove Ganglia from the list here. I replaced with Graphite, but open to leaving it blank or picking some other example.

Fix for overlapping text in feature item headings.

Only happens in certain window sizes.

Added /-/reload endpoint for Prometheus and Alertmanager https://github.com/prometheus/alertmanager/issues/353 (#647)

Prometheus code ref.: https://github.com/prometheus/prometheus/blob/master/web/web.go#L208

Add two exporters by Kumina

Add nginx-vts-exporter