docs(filter): point server_middleware to hexo-server README

[VibhorGautam]

Closes #2325.

As noted in #2325, server_middleware is provided by hexo-server, not Hexo core, so the example on the api/filter page is misleading: it does not run unless hexo-server is installed.

Following the suggestion in the issue, this trims the section to a short note that the filter comes from hexo-server and links to its README for the implementation example. The example itself is added in hexojs/hexo-server#366, so that PR should land first (the README anchor depends on it).

Scope: English page only. The translated pages (ja, ru, zh-cn, zh-tw, pt-br, ko, es, th) still carry the old inline example and can be synced separately.

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Point the README link at an existing example

The new note sends readers to https://github.com/hexojs/hexo-server#middleware, but I checked the linked hexo-server README on master and it only has Installation, Usage, Options, and License headings, so the #middleware anchor does not exist and there is no implementation example there. When users follow this documentation they land on the top of the README without the promised server_middleware guidance; this should either keep the example here or link to a committed page/PR once that Middleware section exists.

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