tools: Deprecate encrypt_secret.py, document zuul-client encrypt

Now that zuul-client's encrypt subcommand covers the same
functionalities as encrypt_secret.py, add a deprecation
message when running the script. Document the zuul-client
encrypt command in the doc section about secrets.

Change-Id: Id5437ffbb688cb80b2744db3beeaa28c97080d90
Depends-On: https://review.opendev.org/765313

doc/requirements.txt

@@ -5,4 +5,5 @@ sphinxcontrib-programoutput
 sphinx-autodoc-typehints
 sphinxcontrib-openapi>=0.4.0
 reno>=2.8.0 # Apache-2.0
+zuul-client
 zuul-sphinx

doc/source/discussion/encryption.rst

@@ -55,9 +55,7 @@ encryption tag may be a list rather than a scalar.  For example:
           - HdWDS9lCBaBJnhMsm/O9tpzCq+GKRELpRzUwVgU5k822uBwhZemeSrUOLQ8hQ7q/vVHln
             ...
 
-Zuul provides a standalone script to make encrypting values easy; it
-can be found at `tools/encrypt_secret.py` in the Zuul source
-directory.
-
-.. program-output:: python3 ../../tools/encrypt_secret.py --help
+The `zuul-client utility <https://zuul-ci.org/docs/zuul-client/>`_` provides a
+simple way to encrypt secrets for a Zuul project:
 
+.. program-output:: zuul-client encrypt --help

tools/encrypt_secret.py

@@ -196,4 +196,9 @@ def main():
 
 
 if __name__ == '__main__':
+    print(
+        "This script is deprecated. Use `zuul-client encrypt` instead. "
+        "Please refer to https://zuul-ci.org/docs/zuul-client/ "
+        "for more details on how to use zuul-client."
+    )
     main()