deprecate Astro.canonicalURL & add Astro.url

[FredKSchott]

What kind of changes does this PR include?

• New or updated content

Description

• Docs PR for RFC Create "Astro.url" & Deprecate "Astro.canonicalURL" roadmap#232
• Marks Astro.canonicalURL as deprecated
• Adds new documentation for Astro.url

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[sarah11918]

Although I don't know how much longer we'll maintain the migration guide, I think it would be good to also make a note of it in the [beta 1.0 section[(https://docs.astro.build/en/migrate/#astro-10-beta), too.

[FredKSchott]

Done! Added a new section to the migration guide for Astro v1 RC

[delucis]

Thanks @FredKSchott — made some suggestions!

[delucis]

Do we want to call out that this of most use with SSR? Push folks towards Astro.url for most common uses?

Suggested change

	`Astro.request` is a standard [Request](https://developer.mozilla.org/en-US/docs/Web/API/Request) object. It can be used to get the `url`, `headers`, `method`, and even body of the request.
	`Astro.request` is a standard [Request](https://developer.mozilla.org/en-US/docs/Web/API/Request) object. It can be used to get the `url`, `headers`, `method`, and even body of the request. This is most useful for sites using [SSR](/en/guides/server-side-rendering/) that want to handle custom requests.

[FredKSchott]

I don't think this is true! It should be equally useful for both SSR and SSG.

[delucis]

One more comment arising out of our docs triage: it’s quite possible we want to start throwing stuff in the RC migration guide before Astro.url ships. Do you want to pull the migration guide stuff into a separate PR so we can get it merged? Then we can start adding content like for the <Markdown /> component (#1045) without waiting for Astro.url.

[FredKSchott]

@delucis sure thing! Feel free to pull the migration guide out of my PR into a separate PR, and then I'll rebase mine off of main once it's merged.

[delucis]

OK, merged the stub migration guide and updated this PR so it’s synced with main, should be good to handle these suggestions however you see fit and merge once Astro.url ships.

[FredKSchott]

Responded to all comments! Thank you @sarah11918 and @delucis for the great feedback.

Merging this without approvals only because feedback was generally positive and I won't be online much tomorrow to merge this then. Happy to respond to any comments still remaining in a follow-up PR!