"usage-examples" style documentation

[uliska]

Currently we have usage-examples subdirectories that contain self-contained LilyPond files demonstrating the capabilities of the package. Depending on the author these are surrounded by more or less comprehensive textual documentation.

My suggestion is to restrict these usage-examples to mere score examples with some metadata only in a \header block. If compiled standalone this can print basic titles, but usually the file will be referenced by a Markdown file and the resulting score excerpt integrated in the output.

The usage-examples files can also be used to create a system of regression tests.

Blocks #26