DOC: Restructure documentation

[scottclowe]

• Split user guide up so each section has its own page.
• Change order of usage guide contents.
• Fix source part of path from appearing in URL.
• Expand section on line continuation.
• Add citation to Usage Guide.

[codecov]

Codecov Report

Base: 28.07% // Head: 28.07% // No change to project coverage 👍

Coverage data is based on head (766f090) compared to base (5bf4cf1).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #316   +/-   ##
=======================================
  Coverage   28.07%   28.07%           
=======================================
  Files          45       45           
  Lines        5346     5346           
  Branches     1309     1309           
=======================================
  Hits         1501     1501           
  Misses       3593     3593           
  Partials      252      252           

Flag	Coverage Δ
unittests	28.07% <ø> (ø)

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