[docs] deprecate build.split and build.excludeMiddleware in config ref

[sarah11918]

…ference

Changes

Updates the configuration reference docs for the two build options being deprecated in favour of new adaptor config options.

• removes @version (is that the right thing to do?) and adds the @deprecated tag
• provides a short description that mentions the adaptor config that replaces it, links to the adapter reference page, and also links to our list of SSR adapter guides and suggests reading those for docs on the new config options.

Testing

No tests, and you must absolutely vet this syntax!

Docs

It's all docs!

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