Replace docs references to PUT with POST (#14270)

The operations are handled identically, but ~85% of the references were
POST, and having a mix of PUT and POST was a source of questions.

A subsequent commit will update the internal use of "PUT" such as by
the API client and -output-curl-string.

website/content/api-docs/system/plugins-catalog.mdx

@@ -100,7 +100,7 @@ supplied name.
 
 | Method | Path                               |
 | :----- | :--------------------------------- |
-| `PUT`  | `/sys/plugins/catalog/:type/:name` |
+| `POST` | `/sys/plugins/catalog/:type/:name` |
 
 ### Parameters
 
@@ -140,7 +140,7 @@ supplied name.
 ```shell-session
 $ curl \
     --header "X-Vault-Token: ..." \
-    --request PUT \
+    --request POST \
     --data @payload.json \
     http://127.0.0.1:8200/v1/sys/plugins/catalog/secret/example-plugin
 ```