move snippets to their own subdirectories
remove "custom-engravers" category

I've decided that the snippets there fit the "notation-snippets"
category.  Being an engraver or a function, or something else
altogether, shouldn't be a basis for categories.
change "LilyJAZZ" to a more generic "custom-music-fonts" category
add README about general-tools category
add new categories to README
misc updates in README
rename 'contributors-guide.md' to 'contributing.md'
new snippet templates, update instructions
update debugging-layout to new, includeable structure
update general-tools to new, includeable structure
change version-predicates example a bit
update snippets to new, includeable structure
notation snippets: reorganize

I've removed all duplicated descriptions.  In my opinion it doesn't
make sense to copy-and-paste so much.  I agree that the description
should be visible to the reader, but i believe we should do it in
another way: include both the description and the image from the
example file in something like the README.md.  Unfortunately, this
may not be possible with GitHub's interface - that's something that
we should do when creating a dedicated web frontend.

I also reverted the scaling-stencils snippet to its previous state,
because this particular snippet isn't worth being divided into
definition and example.  And most importantly, the function that
was defined was not generic - it had ly:flag::print hardcoded,
so it wouldn't work with anything else.