GitLab

The main restructuring thrust is nearing completion, so let's get rid
of the `restructuring` namespace.

Use coqdoc's header feature to make it easier to find key concepts and
improve/tweak a few comments.

It's faster to just compile the new stuff. For new developments,
that's sufficient.

Add the `htmlpretty` target to the Makefile to generate prettier documentation,
based on the CoqdocJS project.

     https://www.ps.uni-saarland.de/~ttebbi/coqdocjs/

     https://github.com/tebbi/coqdocjs

Many thanks to Tobias Tebbi for creating CoqdocJS.

Obviously incomplete, but a start.

First housekeeping steps...