Add user interview with showmax

Added alts to images and explicitly formatted external links

https://github.com/prometheus/docs/pull/421

The tarball now comes with a sample config included, so wouldn't
actually crash anymore. Instead, say that we should configure it first.

Add Mirth exporter to third-party exporters list

Add Youtube's Doorman to direct integrations

meta description had a typo

Corrected the spelling of "efficient" in the meta description

Add FAQ about TLS and authentication.

So far the requests against the Github API to fetch release information
has been made unauthenticated, making them subject to a rate-limit of 10
requests per hour per IP. As the build process is executed on Travis CI,
this results in many build failures.

The requests are now made with a personal access token, in the form of
GITHUB_AUTHENTICATION="-u prombot:<oauth token>". The standard printing
in the Makefile has been removed in order to not expose the token.

Explain when (not) to use the Pushgateway and why

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