add syntax for checking versions
docs for ninja_required_version

(And some extra docs for top-level variables in general.)
depfile helper: check errors on writing .d files

Closes #492 and #493.
say where the default value for -j comes from

This is a FAQ.
move from asciidoc html to asciidoc -> docbook -> html

As best as I can tell this is the easiest way to customize
the asciidoc HTML output.
rearrange env variable section layout
make doc xrefs link to section headers
add versioning notes to features in the manual

This helps guide which ninja version to depend on.