Documentation/process/maintainer-pgp-guide: Replace broken link to PGP path finder

PGP pathfinder[1], which is suggested for finding a trust path to
unknown PGP keys by 'maintainer-pgp-guide.rst', is not working now.
This commit replaces it with other available tools.

[1] https://pgp.cs.uu.nl/

Signed-off-by: SeongJae Park <sjpark@amazon.de>
Reviewed-by: Konstantin Ryabitsev <konstantin@linuxfoundation.org>
Link: https://lore.kernel.org/r/20210812095030.4704-2-sj38.park@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/process/maintainer-pgp-guide.rst

@@ -944,12 +944,11 @@ have on your keyring::
     uid           [ unknown] Linus Torvalds <torvalds@kernel.org>
     sub   rsa2048 2011-09-20 [E]
 
-Next, open the `PGP pathfinder`_. In the "From" field, paste the key
-fingerprint of Linus Torvalds from the output above. In the "To" field,
-paste the key-id you found via ``gpg --search`` of the unknown key, and
-check the results:
-
-- `Finding paths to Linus`_
+Next, find a trust path from Linus Torvalds to the key-id you found via ``gpg
+--search`` of the unknown key.  For this, you can use several tools including
+https://github.com/mricon/wotmate,
+https://git.kernel.org/pub/scm/docs/kernel/pgpkeys.git/tree/graphs, and
+https://the.earth.li/~noodles/pathfind.html.
 
 If you get a few decent trust paths, then it's a pretty good indication
 that it is a valid key. You can add it to your keyring from the
@@ -962,6 +961,3 @@ administrators of the PGP Pathfinder service to not be malicious (in
 fact, this goes against :ref:`devs_not_infra`). However, if you
 do not carefully maintain your own web of trust, then it is a marked
 improvement over blindly trusting keyservers.
-
-.. _`PGP pathfinder`: https://pgp.cs.uu.nl/
-.. _`Finding paths to Linus`: https://pgp.cs.uu.nl/paths/79BE3E4300411886/to/C94035C21B4F2AEB.html