Rename "X guidelines" docs to "Writing X"

Remove the popular option as it only included a single item.

Simplify title of "Exporters and third-party integrations"

"Exporters and integrations" is easier to read in the menu, doesn't wrap
lines, and hopefully means the same thing to people.

The meanings of "Client library guidelines" and "Exporter guidelines" are not
immediately clear. For example, it could mean the usage of client
libraries or exporters. I think "Writing client libraries" and "Writing
exporters" makes this clearer.

Link to alert rule conversion script

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.