chore: Adopt Dune package management

[sabine]

This is the reopen of #3132.

From #3132 (review):

The following changes are required:

• Update .github/workflows/scrape_changelog.yml
• Update README.md, section “Getting Started”
• Do not use developer preview in ci.yml the same build configuration must be used there, in Dockerfile and locally
• Fix make docker build under Ubuntu 25.04
• Remove file ocamlorg.opam

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General concerns that haven't been fully decided / documented:

• To what extent will we support opam workflows on OCaml.org in the future? (@cuihtlauac has explored and considers it feasible)
• The opam-repository pin update scenario: What do we need to communicate about this to contributors, how exaclty do we do this as maintainers? (e.g. have a section in contributor docs about "after rebase on main, my branch doesn't build anymore" and mention that this could be because the dependencies have beeen updated, with example of error message and steps to fix that - deleting dune.lock)

Remaining simple todos::

• we need to use a stable version of Dune on all GitHub Actions Workflows, in the Dockerfile, and for the local build / install (Makefile)
• as a smoke test, we create a new daily(?) GitHub Action that uses Developer Preview to build ocaml.org (this intends to benefit Dune Developer Preview)