docs: update dubbo-proxy doc

[sheharyaar]

Description

Fixes #10648

The PR update the docs for dubbo-proxy with the following information :

• Only Map<String, Object> is supported
• Only hessian2 is supported for serialization, so the user must set this in dubbo config of their applications
• The data to the user program can be passed as header

Checklist

• I have explained the need for this PR and the problem it solves
• [] I have explained the changes or the new features added to this PR
• [] I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[shreemaan-abhishek]

LGTM