tools: add docserve target

Spaws a webserver serving the docs. Helpful to circumvent the OS
permission limitations or to access the docs form a different device on
the local network.

PR-URL: https://github.com/nodejs/node/pull/33221


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>