docs(providers): add provider stubs and support matrix

[khuyentran1401]

Reference Issues or PRs

Fixes #629.

The support matrix links to a Hetzner page that #679 introduces, so #679 should merge first to keep the broken-link check green.

What does this implement/fix?

• Adds a Providers section with a support matrix landing page that shows which deploy targets are usable today (AWS, Hetzner) and which are planned (GCP, Azure, Local), linking each to its page.
• Adds stub pages for the three planned providers, each carrying a uniform "Planned, not yet implemented" banner that links to its scaffolding and invites contributions.
• Extracts that banner into a shared <PlannedProvider /> component so the three pages cannot drift apart.
• Shortens the AWS page title to "AWS" for sidebar consistency.

Testing

• Did you test the pull request locally?
• Did you add new tests?

Verified locally by building the docs and confirming the Providers section, matrix links, and the shared banner render correctly.

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[andrewfulton9]

This looks good to me. I was able to build and serve it locally and everything works as expected. My only comment is that we will have to stay on top of keeping the providers page up to date with the number of production deploy targets are ready and updating the table but I'm not sure if there is a good way around that.