Links to JSON schema files from AsciiDoc files

[javagl]

Right now, there are some links from ADOC files directly to JSON schema files in the GitHub repo, for example,

The common structure of metadata entities that appear in a tileset is defined 
in link:https://github.com/CesiumGS/3d-tiles/tree/draft-1.1/specification/schema/metadataEntity.schema.json[metadataEntity.schema.json].

The reason is that it should be possible to browse to these files, from the GitHub preview of the ADOC file. But one could make a case that these links should point to the "JSON Schema Reference" in the single-page HTML version.

In any case, we should identify these links, and maybe try to make them point to

• the actual JSON file on GitHub
• the inlined JSON in the single-page HTML

consistently in the specification. (This should be possible using the ifdef-trickery that is already used for other links - it may be a bit clumsy, but there should not be sooo many of these links, and therefore, it could be manageable)

[javagl]

The schema links have been modified accordingly, e.g. with

The common structure of metadata entities that appear in a tileset is defined in 
ifdef::env-github[]
link:https://github.com/CesiumGS/3d-tiles/tree/main/specification/schema/metadataEntity.schema.json[`metadataEntity.schema.json`].
endif::[]
ifndef::env-github[]
<<reference-schema-metadataentity,`metadataEntity.schema.json`>>.
endif::[]

so that they point to GitHub when viewn on GitHub, and the the inlined HTML version in the HTML version.

If there are cases where this pattern has not been applied, they can be brought up in own issues.