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.

Relative links always need to point to one level below the current path,
as nanoc path end on a trailing slash.