Produce v3.1 of the LFE Docs Site

[oubiwann]

The work on the LFE docs site continues, carried over from v3.0.

This work maintains the same goals as the v3.0 release:

• Provide a usable site-generating infrastructure (tools and procedures) for docs contributors
• Produce a clean, easy-to-navigate site that is the single source of truth for all LFE docs, regardless of where the bits actually live.
• Maintain versioned docs so that those who are stuck using older versions of LFE and/or Erlang are supported (to the extent that docs existed when their chosen version was released).

To do this, a great deal of work is necessary. The major categories of this work are represented by the tickets listed below, each with their own tasks and/or supporting tickets:

• Look and Feel - Look and Feel #96
• Unification of Disparate Content - Unification of Disparate Content #97
• Guidelines for Individual Projects - Guidelines for Individual Projects #70
• Add Missing Content - Add Missing Content #75
• Revamp Old Content - Revamp Old Content #98
• Infrastructure Updates - Infrastructure Updates #102

[oubiwann]

We're going to ditch the docs site with all serious content going into LFE books, and general docs content going to lfe.io/docs