remove "server_middleware" from "api/filter"

[uiolee]

Check List

• I have already read README.
• I have already searched existing issues.
• I have already searched existing pull requrests.

Feature Request

"server_middleware" is not executed in "hexo", but in "hexo-server"

Should we remove it from the "api/filter" documentation?

Additional context

No response

[yoshinorin]

IMHO, I think it would be better to add implementation examples to the hexo-server's README and include a link to that README from the api/filter documentation.