nixos-render-docs: don't use markdown-it RendererProtocol

our renderers carry significantly more state than markdown-it wants to
easily cater for, and the html renderer will need even more state still.
relying on the markdown-it-provided rendering functions has already
proven to be a nuisance, and since parsing and rendering are split well
enough we can just replace the rendering part with our own stuff outright.

this also frees us from the tyranny of having to set instance variables
before calling super().__init__ just to make sure that the renderer
creation callback has access to everything it needs.

pkgs/tools/nix/nixos-render-docs/src/nixos_render_docs/docbook.py

@@ -32,14 +32,13 @@ class Heading(NamedTuple):
     partintro_closed: bool = False
 
 class DocBookRenderer(Renderer):
-    __output__ = "docbook"
     _link_tags: list[str]
     _deflists: list[Deflist]
     _headings: list[Heading]
     _attrspans: list[str]
 
-    def __init__(self, manpage_urls: Mapping[str, str], parser: Optional[markdown_it.MarkdownIt] = None):
-        super().__init__(manpage_urls, parser)
+    def __init__(self, manpage_urls: Mapping[str, str]):
+        super().__init__(manpage_urls)
         self._link_tags = []
         self._deflists = []
         self._headings = []