doc: document `make docopen`

Documented `make docopen` as a way to read documentation in the browser.

PR-URL: https://github.com/nodejs/node/pull/19321


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>