nixos-render-docs: indent and embolden list item heads in manpages

this matches what html outputs do more closely, and feels like it'll be
easier to read because it looks less like just another paragraph.

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

···
         lst.first_item_seen = True
         head = "•"
         if lst.next_idx is not None:
-            head = f" {lst.next_idx}."
+            head = f"{lst.next_idx}."
             lst.next_idx += 1
         return (
             f'{maybe_space}'
             f'.RS {lst.width}\n'
-            f"\\h'-{lst.width}'{man_escape(head)}\\h'{lst.width - len(head)}'\\c"
+            f"\\h'-{len(head) + 1}'\\fB{man_escape(head)}\\fP\\h'1'\\c"
         )
     def list_item_close(self, token: Token, tokens: Sequence[Token], i: int, options: OptionsDict,
                         env: MutableMapping[str, Any]) -> str: