the nixos manual contains enough examples to support them as a proper
toc entity with specialized rendering, and if in the future the nixpkgs
wants to use nixos-render-docs we will definitely have to support them.
this also allows us to restore some examples that were lost in previous
translation steps because there were too few to add renderer support
back then.
this adds support for structural includes to nixos-render-docs.
structural includes provide a way to denote the (sub)structure of the
nixos manual in the markdown source files, very similar to how we used
literal docbook blocks before, and are processed by nixos-render-docs
without involvement of xml tooling. this will ultimately allow us to
emit the nixos manual in other formats as well, e.g. html, without going
through docbook at all.
alternatives to this source layout were also considered:
a parallel structure using e.g. toml files that describe the document
tree and links to each part is possible, but much more complicated to
implement than the solution chosen here and makes it harder to follow
which files have what substructure. it also makes it much harder to
include a substructure in the middle of a file.
much the same goes for command-line arguments to the converter, only
that command-lined arguments are even harder to specify correctly and
cannot be reasonably pulled together from many places without involving
another layer of tooling. cli arguments would also mean that the manual
structure would be fixed in default.nix, which is also not ideal.
pandoc drops .title classes when rendering to docbook, so these are
effectively just paragraphs anyway. without support for including them
in a table of contents the complexity of parsing them in
nixos-render-docs won't be warranted.
it's really easy to accidentally write the wrong systemd Exec* directive, ones
that works most of the time but fails when users include systemd metacharacters
in arguments that are interpolated into an Exec* directive. add a few functions
analogous to escapeShellArg{,s} and some documentation on how and when to use them.
