chore(docs): prevent readme overwrite (#10170)

# Which Problems Are Solved

To generate the docs, we rely on a protoc plugin to generate an openAPI
definition from connectRPC / proto.
Since the plugin is not available on buf.build, we currently download
the released version. As the tar contains a licence and a readme, this
overwrote existing internal files.

# How the Problems Are Solved

Download and extract the plugin in a separate folder and update
buf.gen.yaml accordingly.

# Additional Changes

None

# Additional Context

relates to #9483

docs/buf.gen.yaml

@@ -9,7 +9,7 @@ plugins:
       - allow_delete_body
       - remove_internal_comments=true
       - preserve_rpc_order=true
-  - local: ./protoc-gen-connect-openapi
+  - local: ./protoc-gen-connect-openapi/protoc-gen-connect-openapi
     out: .artifacts/openapi3
     strategy: all
     opt:

docs/plugin-download.sh

@@ -1,5 +1,6 @@
 echo $(uname -m)
-
+mkdir protoc-gen-connect-openapi
+cd ./protoc-gen-connect-openapi/
 if [ "$(uname)" = "Darwin" ]; then
   curl -L -o protoc-gen-connect-openapi.tar.gz https://github.com/sudorandom/protoc-gen-connect-openapi/releases/download/v0.18.0/protoc-gen-connect-openapi_0.18.0_darwin_all.tar.gz
 else