Fixes prometheus/prometheus#3451
Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

We support the SignalFx metric proxy as Prometheus Remote Storage.

Added a draft of a "first steps" guide. Very rough, could be expanded or
contracted and very open to feedback. I added the scaffolding for guides
as an example of how they might work but not attached to their menu
placement, nav icon, etc, etc.

A faster and easier-to-use kafka exporter

Add Cortex & CrateDB as additioanl remote storage integrations.

Thanks to @juliusv for mentioning them in irc.

Neither of these anchor tags work atm.

Escape the links in the same way known-working FAQ.md does.
See the link to configuration#<scrape_config>:
https://prometheus.io/docs/introduction/faq/#why-don-t-the-prometheus-server-components-support-tls-or-authentication-can-i-add-those

Remove done items from the roadmap

We have too many now.

Prometheus 2.0 announcement blog post

Versioned documentation

This reverts commit af282717.

Add Fabian to support

Documented global defaults for email_config

The security page was describing our general philosophy and "concepts"
was the closest match for that. Integrations are part of Instrumenting
now.