doc: document make docopen

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

PR-URL: #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>

BUILDING.md

@@ -206,6 +206,15 @@ To read the documentation:
 $ man doc/node.1
 ```
 
+If you prefer to read the documentation in a browser,
+run the following after `make doc` is finished:
+
+```console
+$ make docopen
+```
+
+This will open a browser with the documentation.
+
 To test if Node.js was built correctly:
 
 ```console