Documentation to build gnatdoc

[johnperry-math]

This ticket requests detailed, up-to-date documentation on how to build gnatdoc from source. Problems I've encountered while trying to do this:

• Building gnatdoc from alire works, but produces a non-working binary. (See tickets gnatdoc crashes on html output #35 and gnatdoc crashes when trying to print odf or xml output #36.)
• Building gnatdoc from cloned github repository seems non-trivial:
  • It relies on a Makefile rather than alire or a top-level gpr file.
  • make fails because it cannot find certain dependencies.
  • Nothing in the documentation tells you where to find the dependencies.
  • One of the dependencies, vss, was recently split into subprojects, and a small amount of digging around leaves me unclear as to:
    • where I can obtain the ones needed for gnatdoc
    • how I should install them -- presumably in the subprojects folder