DOC: Self-hosted & multiversion documentation overhaul

[oesteban]

Following after nipreps/niworkflows#656

[oesteban]

Pending: updating gh-pages accordingly - will do later

[codecov-commenter]

Codecov Report

Merging #243 (d524de6) into master (62160d0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #243   +/-   ##
=======================================
  Coverage   83.06%   83.06%           
=======================================
  Files          25       25           
  Lines        1836     1836           
  Branches      208      208           
=======================================
  Hits         1525     1525           
  Misses        280      280           
  Partials       31       31           

Flag	Coverage Δ
travis	83.06% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 62160d0...d524de6. Read the comment docs.

[effigies]

This is going to make it trickier to rerender older versions at need. Or rather, it will create a watershed where we do one thing to render before and another thing to render after. Even if we can't get a single "correct" result now, I would suggest that we leave this rule unchanged and create a new rule like htmlbranch.

[oesteban]

Since deployment is now decoupled (one needs to actively checkout gh-pages and add/edit contents) I don't think it has much of an effect.

What probably would make sense is to change the name of the variable - OUTDIR would be much more adequate, and the default value would be "html" for consistency with the former make html target. WDYT?