Add hosting platform field and filter to SSG and CMS lists

[matbrgz]

Note

Stacked PRs — merge in order: #1104 → #1117 → #1118. These three touch the same entry files, so each branch is built on top of the previous one (PRs from a fork can't target another PR's branch, so all three target main).
This PR: top of the stack, builds on #1117. Review only the last commit (diff vs #1117).

Adds a new hosting frontmatter field and a Hosting filter on the Site Generators and Headless CMS pages, so people can find tools with official support for Vercel, Cloudflare, Netlify, Railway, AWS, GCP and Azure. The field is also shown on cards and tool pages.

hosting:
  - Vercel
  - Netlify

What counts as "supported"

A platform is listed only when at least one official source exists:

1. the platform's framework preset / auto-detection list (Vercel frameworks.ts, Netlify build-info, Cloudflare Pages presets);
2. a guide in the platform's own docs (Cloudflare framework guides, Netlify frameworks, Railway guides, AWS Amplify user guide, Azure Static Web Apps / App Service, Firebase Hosting/App Hosting, Cloud Run);
3. an official one-click template (Railway: vendor-published or verified only);
4. a dedicated deploy guide or first-party adapter/preset in the tool's own docs.

AWS = Amplify Hosting, GCP = Firebase Hosting/App Hosting or Cloud Run. For CMSs the CMS itself must be deployable on the platform (frontend-only starters don't count).

Initial data covers 75 entries (e.g. Hugo, Astro, Next.js, Eleventy, Strapi, Directus, Payload, Ghost). Any static output can of course be served from any static host; this field is about official support, and entries without data simply don't show the field. More can be added by contributors over time.

Note: src/site/headless-cms.njk is also touched by #1104 (language filter) in a different spot of the same filter bar.

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[matbrgz]

Heads-up for reviewers: this is the third of three stacked PRs. Please review and merge in this order:

1. Add language filter to headless CMS list #1104 – language filter for the headless CMS list
2. Remove dead entries and fix outdated links in SSG and CMS lists #1117 – dead-entry cleanup and link fixes
3. Add hosting platform field and filter to SSG and CMS lists #1118 (this PR) – hosting platform field and filter

This branch is built on top of #1117, so please merge #1104 and #1117 first. Until then, the diff here also includes their commits. To review only this PR's changes (the last commit), use this comparison: webminas/jamstack.org@audit-dead-links...add-hosting-platforms