docs: document Environment API breaking changes

[matthewp]

Description (required)

Updates the v6 guide with breaking changes from the Environment API refactor

Related issues & labels (optional)

For Astro version: 6.x. See astro PR #14306.

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[ArmandPhilippot]

Thanks Matthew, this looks great. I left a few suggestions, mostly about headings/code snippet consistency.

My main concern is the "RouteData.generate() Removed" section. I don't think we document RouteData directly, but I can see it used in the Integrations page to transmit properties. Looking at the implementation PR, it seems generate() still exist under another format. So, maybe this is the change we want to document here? (but maybe I'm wrong, the implementation PR contains a lot of changes, I haven't checked all the details yet 😅 )

Also, while I was quickly looking at the implementation PR, I noticed we have two new properties for setAdapter(): we'll need to document them in the Adapter API guide under Building an adapter. We now have a subheading for each available prop, so we'll need to add devEntrypoint and entryType there.