New module/section: contact end-game & bear-off technique

From tester feedback (jholmertz, Discussion thread on training areas).

Two related end-game topics:
- Bringing the last checkers into the home board while the opponent still has
checkers there, minimising the risk of leaving blots.
- Bearing off while the opponent still has checkers in your home board
(bear-off under contact).

This is checker/move technique rather than a cube topic. Note it would require
the single-checker board rendering that is currently deferred to "step 2" of
the renderer (the same work needed for Half-Crossover), so it depends on that
being built first.

Status: collecting ideas / needs didactic basis and the single-checker
renderer before proceeding.

Note: check whether this can be combined with related issues into a shared
theme — there may be overlap with the existing "Bearing off" idea, depending
on whether that one covers contact-free bear-off or contact bear-off.

Start with the training areas discussion thread and review the existing "Bearing off" idea to identify overlap. The single-checker board renderer, deferred to renderer step 2 and also needed for Half-Crossover, must be available first. Done means a didactic contact end-game module covering both topics, with the required rendering support.

Written by the indexing model from the issue text.