docs: add http-dubbo docs

[ShenFeng312]

Description

Fixes https://github.com/apache/apisix/issues/11067

Add http-dubbo docs

Checklist

• [x] I have explained the need for this PR and the problem it solves
• [x] I have explained the changes or the new features added to this PR
• [ ] I have added tests corresponding to this change
• [x] 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)

[kayx23]

@ShenFeng312 Please address the remaining review comments to move this forward, thanks.