add new fusion landing page

[mirnawong1]

this pr adds a new fusion landing page to orient users by letting them know:

• added new availabilitypage that explains where Fusion is available with clear bullets and preview lifecycle.
• created/expanded shared snippet website/snippets/_fusion_availability.md to centralize availability details and an environment/tools/features matrix.
• updated fusion landing page per @dataders feedback to feature cards linking to the new Availability, Supported features, Install, and Upgrade pages.
• added navigation entry for the new page in website/sidebars.js
• kept or expanded the “Supported features” content and added the features table.

this was raised as the fusion docs are rcih in information but it's not clear where users should start their fusion-info journey. so this serves to orient users.

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🚀 Deployment available! Here are the direct links to the updated files:

• https://docs-getdbt-com-git-fusion-landing-dbt-labs.vercel.app/docs/about-dbt-extension
• https://docs-getdbt-com-git-fusion-landing-dbt-labs.vercel.app/docs/cloud/about-develop-dbt
• https://docs-getdbt-com-git-fusion-landing-dbt-labs.vercel.app/docs/dbt-versions/product-lifecycles
• https://docs-getdbt-com-git-fusion-landing-dbt-labs.vercel.app/docs/dbt-versions/release-notes
• https://docs-getdbt-com-git-fusion-landing-dbt-labs.vercel.app/docs/dbt-versions/upgrade-dbt-version-in-cloud
• https://docs-getdbt-com-git-fusion-landing-dbt-labs.vercel.app/docs/fusion/about-fusion-install
• https://docs-getdbt-com-git-fusion-landing-dbt-labs.vercel.app/docs/fusion/about-fusion
• https://docs-getdbt-com-git-fusion-landing-dbt-labs.vercel.app/docs/fusion/fusion-availability
• https://docs-getdbt-com-git-fusion-landing-dbt-labs.vercel.app/docs/fusion/fusion
• https://docs-getdbt-com-git-fusion-landing-dbt-labs.vercel.app/docs/fusion/install-fusion
• https://docs-getdbt-com-git-fusion-landing-dbt-labs.vercel.app/docs/fusion/new-concepts
• https://docs-getdbt-com-git-fusion-landing-dbt-labs.vercel.app/docs/fusion/supported-features
• https://docs-getdbt-com-git-fusion-landing-dbt-labs.vercel.app/docs/introduction
• https://docs-getdbt-com-git-fusion-landing-dbt-labs.vercel.app/docs/running-a-dbt-project/using-threads
• https://docs-getdbt-com-git-fusion-landing-dbt-labs.vercel.app/guides/coalesce-fusion-platform-qs
• https://docs-getdbt-com-git-fusion-landing-dbt-labs.vercel.app/guides/fusion-qs

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Project	Deployment	Preview	Updated (UTC)
docs-getdbt-com	Ready	Preview	Oct 27, 2025 4:05pm

[mirnawong1]

condensing content - table and icons difficult to read in these bullets.

[mirnawong1]

fixing broken links too!

[matthewshaver]

@mirnawong1 This looks amazing and I think it will be a huge boost to Fusion adoption and reducing confusion. Thank you!