More markup cleanups/fixes.

Fix logo and docs menu alignment.

Instead of moving the logo to the right to align it with the docs
side-nav, remove the extraneous left-padding on the side-nav. Then the
logo is aligned with the text on all pages again.

Add download section

This adds a new download section to the page with list for all available
precompiled versions for officially maintained components. The release
information are fetched via the Github API and rendered as static list.

Currently only the latest release and the newest pre-release (if
available) are show. This could be extended later. In order to keep the
list usable besides the massive list of prebuilt binaries, a javascript
os/arch filter has been added, defaulting to popular versions.

The following changes have been made on the way:

* Ensure all pages have a footer.
* Remove duplicated jquery inclusion.
* Use CSS to define text case of jumbotron buttons.
* Add Download section to the header.
* Reduce Github link to just the logo to save space.
* Update Getting Started and Installing sections.

Abridge blog posts in index view.

Fix and improve some links.

Fix broken references to html anchors.

Improve landing page layout on smaller screens.

- The jumbotron text was not correctly centered between 750px and 970px
  width. Fixed by removing unnecessary HTML.
- Feature items flipped from 4 per row to 1 per row, without any 2-per row
  variant for medium-sized screens. Added a 2-per-row variant.
- The feature items had a fixed height which looked too tall on the
  large end of XS widths (one long item per row). Removed the fixed
  height and let the height adjust itself automatically.

Remove title case in exporter and clientlib guidelines.

We have opted for not using title case in headers in Prometheus docs.

Word-wrap exporter and clientlib guideline docs.

This is to keep it consistent with the rest of our docs and to make
subsequent diffs less crazy.

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.