Fixed broken/outdated docs links (#138)

.github/workflows/pants.yaml

@@ -1,7 +1,7 @@
 # Copyright 2020 Pants project contributors.
 # Licensed under the Apache License, Version 2.0 (see LICENSE).
 
-# See https://pants.readme.io/docs/using-pants-in-ci for tips on how to set up your CI with Pants.
+# See https://www.pantsbuild.org/2.21/docs/using-pants/using-pants-in-ci for tips on how to set up your CI with Pants.
 
 name: Pants
 
@@ -45,7 +45,7 @@ jobs:
         # Note that named_caches and lmdb_store falls back to partial restore keys which
         # may give a useful partial result that will save time over completely clean state,
         # but will cause the cache entry to grow without bound over time.
-        # See https://pants.readme.io/docs/using-pants-in-ci for tips on how to periodically clean it up.
+        # See https://www.pantsbuild.org/2.21/docs/using-pants/using-pants-in-ci for tips on how to periodically clean it up.
         # Alternatively you change gha-cache-key to ignore old caches.
     - name: Check BUILD files
       run: |