Add Client Lib Guidelines and Exporter Guidelines

* Use relative links for all documentation pages.
* Add links were missing.
* Mark all function, metric and label names as such.
* Remove usage of "No-Label metric" as it was very hard to read.
* Adjust headings for an easier toc navigation and group related parts.

Remove dead link to Boxever website.

Remove third-party blackbox exporter

Repository says it is deprecated and links to the official blackbox exporter.

Update routing tree location

Move GitHub star button to "Open Source" section.

Routing tree editor is now only available as a
link from within the configuration docs for
alertmanager.

Code changes:
- Routing tree CSS is now in a CSS file
- The label set input now draws the route when
"enter" is pressed

Change codebox/toc color from red-ish to gray.

Improve overall website design.

- Thinner and more elegant fonts on landing page.
- Removed "Overview" menu item, shortened "Documentation" to "Docs".
- Replaced jumbotron logo+title with motivational text.
- Removed "View details" buttons on landing page, make features clickable
  instead.
- Introduced testimonial quote section on landing page.
- Introduced "Open Source" section on landing page.
- Refactored overall jumbotron layout/content.
- Removed PromCon 2016 button.
- Changed documentation font to smaller size.
- Adjusted header font sizes and weights.
- Made code blocks look cooler, with orange left border.
- Don't wrap long lines in code blocks.
- Harmonized background colors between pre, code, and toc.
- Aligned the navbar Prometheus logo with the docs menu.

add routing tree editor

Rename heka-exporter and add new heka-exporter

We discuss renaming the old heka-exporter repo to heka-dashboard-exporter to avoid confusion here: https://github.com/docker-infra/heka_exporter/issues/2

Add cachebusting fingerprints to asset files.

This is using http://avdgaag.github.io/nanoc-cachebuster/.

docs.css had to be moved out of /static because the static datasource
treats all files as binary, and thus no textual filters can be applied
(necessary for rewriting the jumbotron background image URL).

Fixes https://github.com/prometheus/docs/issues/369

List unofficial Common Lisp client

https://github.com/deadtrickster/prometheus.cl

add notice for default behavior of using 3GB RAM

Clarify replace relabel behavior with non-matching regex

Fix blog sidenav.

This got broken in https://github.com/prometheus/docs/pull/365

Improve search engine result presentation

- best to keep title length under 55 characters
- descrioption should not be longer than 150-160 characters

Link to community section

Collapse inactive navigation menus

Use javascript to hide all inactive menu items and only show one active
menu item at a time.

Set descriptive front page title