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

Add exporter for Mozilla Observatory

Adding the prometheus exporter for Mozilla Observatory to docs

An extra warning is included as these functions make it easy to
produce metrics that cause duplicate time series data.

Documents prometheus#2279

Tarantool is a fast in-memory database and an application
server for Lua apps.

Homepage: https://tarantool.org

Signed-off-by: Xabier Larrakoetxea <slok69@gmail.com>