chore: migrate preview deployments from Vercel to Cloudflare Pages via GitHub Actions

[scode2277]

This PR:
Replaces Vercel preview deployments with Cloudflare Pages Direct Upload using the refined-cf-pages-action. This enables preview deployments for PRs from forks, as well as move all the CF deployment to go through the github workflows and not anymore with the git integration.

The updates of the PR:

• Add deploy workflow for all branch pushes (main/develop/all other internal branches)
• Add build + deploy split workflows for fork PR previews. 2 workflows are needed as a safety measure so the build runs in an unprivileged context as a protection against forks content / two workflows are needed as a safety measure: the build runs in an unprivileged context with no access to secrets, protecting against malicious fork content. The deploy step only uploads pre-built static files
• Remove vercel.json and all Vercel references from all the files
• Update searchbar-indexing and sitemap-generator to run in CI, as the build now runs on github action runners instead of cf pages

What will change in our usual workflow:

• The only thing is that, with this approach, we will check the logs directly in the Action run instead of the Cloudflare Pages dashboard. The CF dashboard will still show deployment history and URLs, but won't have build output

Closes #280

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[cloudflare-workers-and-pages]

Deploying frameworks with    Cloudflare Pages

Latest commit:	a28e08e
Status:	✅  Deploy successful!
Preview URL:	https://5306471d.frameworks-573.pages.dev
Branch Preview URL:	https://chore-cf-workflows.frameworks-573.pages.dev

View logs

[mattaereal]

It should be already working by now, let's test it out