doc: add link to debugger in process.md

PR-URL: #17522
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>

doc/api/process.md

@@ -379,7 +379,7 @@ terminal programs.
 
 It is important to take note of the following:
 
-* `SIGUSR1` is reserved by Node.js to start the debugger.  It's possible to
+* `SIGUSR1` is reserved by Node.js to start the [debugger][]. It's possible to
   install a listener but doing so will _not_ stop the debugger from starting.
 * `SIGTERM` and `SIGINT` have default handlers on non-Windows platforms that
   resets the terminal mode before exiting with code `128 + signal number`. If
@@ -1971,6 +1971,7 @@ cases:
 [`v8.setFlagsFromString()`]: v8.html#v8_v8_setflagsfromstring_flags
 [Child Process]: child_process.html
 [Cluster]: cluster.html
+[debugger]: debugger.html
 [Duplex]: stream.html#stream_duplex_and_transform_streams
 [LTS]: https://github.com/nodejs/LTS/
 [note on process I/O]: process.html#process_a_note_on_process_i_o