doc: refactor manpage to use mdoc(7) macros

Like man(7), mdoc(7) is a macro package for marking up computer manuals.
The main difference is that mdoc is semantic rather than presentational,
providing authors with a full DSL to abstract page markup from low-level
Roff commands. By contrast, `man` is minimalist and leaves formatting to
the author, who is expected to have a working amount of Roff knowledge.

Therefore the use of `mdoc` for marking up Node's manpage is a decidedly
better choice than bare `man`. Less room is left for error and mandoc(1)
offers very robust error-checking and linting.

PR-URL: #18559
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>