fix(astro:middleware): add docs for astro/middleware imports

[ArmandPhilippot]

Description (required)

Updates the astro:middleware docs to:

• Add two sections for imports available from astro/middleware
  • userDefinedLocales added in feat: experimental i18n routing astro#8974
  • defaultLocale added in fix(routing): default locale when there's a list astro#12151
  • locals added in fix(routing): don't attach locals to request astro#12647 but there is no changeset ... the last v4 version was released in November, so I think it's safe to say v5 here.
• Move createContext and trySerializeLocals from astro:middleware to astro/middleware (not available in the colon version)
• Add defineMiddleware() type and update the description
• Update sequence() code snippet to make it valid and replace _ with context to improve clarity.

Things to consider:

• Is it okay to documented astro/middleware in the same place given the page is named astro:middleware? If so, maybe we should add a sentence to explain when to use one over the other.
• We already document defineMiddleware() and sequence() in astro:middleware imports but the same functions are available from astro/middleware: should they be available from only one version? how to document both without repeating the same thing.

See also withastro/astro#8101 (review) for extra context.

Related issues & labels (optional)

• Suggested label: add new content, improve or update documentation

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[sarah11918]

After some discussion:

astro/middleware in the same place given the page is named astro:middleware?

Yes, let's do this every time there's a regular module associated with the virtual model. Maybe we can have a sentence like "The following are imported from the virtual content module" / "The following are imported from the regular content module" to introduce each code block of imports.

We checked, and there should be a corresponding virtual model for every one of these modules that isn't already covered by its own special API page (e.g. astro/loaders is covered in the Content Loader API page), so keeping these pages named for the virtual modules seems fine.

should they be available from only one version? how to document both without repeating the same thing.

"Should" is something to take up with the framework team! If we're responsible for documenting what is currently, and if both entries would be identical, then it's OK I think to:

• show them in both import code blocks

• put the reference entry in the module that we expect more people are using (I suspect the virtual one?)

• make a section heading in the other module, too, but its only text is to link back to the one written in the other module, maybe like:

   See [`defineMiddleware()`](#definemiddleware) from `astro:middleware`