WIP Add htmlpipeline

Adds https://github.com/burnto/nanoc-html-pipeline by using https://github.com/jch/html-pipeline.

Check how https://github.com/atom/flight-manual.atom.io does it.

Purpose

We can extend the HTMLPipeline with our own stuff, so we can do things like:

<omnibus>
Omnibus stuff
</omnibus>

<source>
Source installation stuff
</source>

and have both info (Omnibus + source) in the same source doc that when built with nanoc, we could toggle it with a button. Something like what the Atom flight manual does. For example, there is info on installing on all three platforms and instructions change with a click of a button:

• Rendered page http://flight-manual.atom.io/getting-started/sections/installing-atom
• Source markdown https://github.com/atom/flight-manual.atom.io/blob/master/content/getting-started/sections/installing-atom.md

What it solves

As I've written in https://gitlab.com/gitlab-org/gitlab-ce/issues/18739#note_25486102

We should avoid splitting Omnibus and source docs in areas that are hard to provide both in the same doc:

• Pages admin docs
  • Omnibus
  • Source
• Redis HA
  • Omnibus
  • Source

We also had to split the Geo docs because they are very hard to follow https://gitlab.com/gitlab-org/gitlab-ee/issues/1908.

added 57 commits

• 0867ba81...8abd16bc - 56 commits from branch master
• 21771e7c - WIP add htmlpipeline

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