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/raw.mdx

@@ -56,7 +56,7 @@ mount system.
 
 | Method | Path             |
 | :----- | :--------------- |
-| `PUT`  | `/sys/raw/:path` |
+| `POST`  | `/sys/raw/:path` |
 
 ### Parameters
 
@@ -85,7 +85,7 @@ mount system.
 ```shell-session
 $ curl \
     --header "X-Vault-Token: ..." \
-    --request PUT \
+    --request POST \
     --data @payload.json \
     http://127.0.0.1:8200/v1/sys/raw/secret/foo
 ```