Astro Middleware documentation

[ematipico]

The platform team is about the add a new feature in astro. The feature is called "Astro middleware".

Few useful links:

• RFC Stage 2, here's a relevant comment from @matthewp that highlights possible use cases;
• RFC Stage 3
• Relevant PR, the description of the PR has instructions of how to install the preview release;

A middleware unlocks two major things:

• do various operations based on the Request and Response, and also modify the Response before being rendered; a user can also return a new Response;
• store contextual information inside a new global called Astro.locals/context.locals (they're the same object), and then render something based on this Astro.locals;

[ematipico]

cc @withastro/maintainers-docs

[sarah11918]

Closing as initial documentation has been released! 🥳

Now, updates can be done by referencing the existing docs page!