Bring back missing preambles (content up to first heading) on package documentation

[sabine]

• upstream patched Experimental --as-json option for the HTML renderer odoc#908 (the --content-only option would omit the preambles, and ocaml.org is currently reconstructing metadata from the URL).
• ocaml-docs-ci pipeline has built documentation for all packages with odoc 2.2.0
• patch ocaml.org (patch needs to be rebased, but otherwise good to go)

[sabine]

Note: It will take around 24-48 hours until all package documentation has been rebuilt. We are falling back to old builds of package documentation in case of error or when the odoc.2.2.0-using package documentation has not been built yet.