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.

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.