(internal) Links are placed in square brackets. They must not contain
whitespaces or word-splitting characters, to be friendly to editor's
"goto file" feature. CamelCase or kebab-case are perfect!

Pages can be placed in directory hierarchy. Path to page is relative to
the root of the project. $root/foo/bar file can be linked from any page
with the [foo/bar] link. Link to the directory (category) itself,
containing "index" file (may be autogenerated), must contain trailing /.
Relative links are not allowed.
