This commit is a first in a series to extract all Prometheus server
specific documentation pages and load them from the prometheus
repository instead.

With this commit it becomes possible to define additional data sources
for content pages. The content will be cloned from the specified
repository. Links in these pages will be normalized to work inside the
docs project. It's assumed the documentation pages include the standard
nanoc config options and set :title and optionall :sort_rank.

Remove me from community.html, replace with Kausal

Propose to add [swagger-stats](https://github.com/slanatech/swagger-stats) to the list of Other third-party utilities. swagger-stats is node.js module that exposes Prometheus metrics for REST API in node.js app.

Fixes #636

* Some updates to the introduction documents

1. Some spelling and grammar updates, settled on US English as that
seemed consistent? Happy to be wrong...

2. Removed a dead link from the FAQ.

3. Tidied up some awkward sentences.

4. Fixed some missing or extra words.

5. Added a couple of links to external tools, etc.

6. Added NOTE formatting when things are an actual node.

added new ec2 discovery role_arn option

Add the aws-health-exporter to the list of exporters.

Promote PromCon 2017 videos in newsbar

Add consul node metadata documentation

PromCon 2017 recap blog post

I've updated the NATS Exporter to be the one maintained by the NATS team, which was released this week. The previous link was to Lovoo's exporter - Markus (maintainer of Lovoo's) is aware of this change and fine with it.