feat: introduce 'Backstage OCaml' by splitting out 'unstable' comms

[sabine]

what	Before	After
OCaml Changelog
Backstage OCaml	didn't exist

Changes:

• Introduced "Backstage OCaml" - A new content category for unstable/experimental communications, separating them from the main OCaml Changelog
• Created three-tier content organization: OCaml Changelog, Backstage OCaml, and Platform Tools Releases. (Refactored Platform Releases out of tools/ood-gen/lib/changelog.ml into tools/ood-gen/lib/platform_tools_release.ml, Introduced tools/ood-gen/backstage.ml, which considers unstable OCaml Platform Releases and posts from data/backstage, while tools/ood-gen/changelog.ml considers stable OCaml Platform Releases and posts from data/changelog)
• Moved all existing posts either under data/changelog/ or data/backstage/ and all the platform tools release announcements under data/platform_tools_releases/.
• Add redirects for former OCaml Changelog posts which are now sorted under Backstage OCaml (Dune Developer Preview, and Infrastructure announcements)
• Updated CONTRIBUTING.md to reflect OCaml Changelog / Backstage OCaml / Platform Tools Releases triad
• Update the OCaml Platform Tool release scraper to work with the new structure

Todo:

• very carefully deal with URL structure and redirects (if we're placing backstage posts under /backstage/, we need to redirect the relevant old posts from /changelog/ to their new URLs!)
• update contributor docs for maintainers who add releases and Backstage OCaml and OCaml Changelog posts
• [ ] place one big feed that aggregates everything (OCaml Changelog, Backstage OCaml, OCaml Planet, Newsletters) under /feed.xml, instead of redirecting to the OCaml Planet (let's do this as a separate issue/task)

After Merge:

• Update social media integrations to distribute Backstage OCaml content under appropriate framing (unstable / experimental / behind the scenes / contributor-relevant)