Explain meta-schema status and support

[handrews]

Every time we fix a bug in place someone complains, so explain somewhere that we do that, what it means, and that it's on purpose. Also say something about what sort of changes would not be fixed in place.

TL;DR: If the meta-schema contradicts the spec, and the fix will not cause problems with users who were complying with reasonable interpretations the spec, then we'll fix it in place. Examples:

• Leaving out a keyword. Adding it in will only cause problems if someone was using the keyword for something else. Most users would prefer the meta-schema to detect improper use
• Egregiously wrong defaults, like the string "true" instead of the boolean true. Defaults are not intended to be automatically written into instances, but if the default is the wrong type then anyone who attempts to use it will have problems.

I can't come up with something that we wouldn't fix in place, as the more dramatically breaking things would probably be caught as soon as people start using it. In which case it's better to fix even a dramatic thing in place. But let's say something about the possibility so people don't completely freak out thinking that we'll just arbitrarily mess with it.

[handrews]

I've added this in a PR on the CONTRIBUTING.md file in the spec repo. If that is approved, we should also add the key points about updating meta-schemas on the web site as well. The web site should have everything a user would need.

[handrews]

We now have a different approach to meta-schema versioning and should explain that.

We should also explain that the unversioned URIs shouldn't be used, and why they do weird HTML things (assuming we get to this before we get the new hosting set up). See json-schema-org/json-schema-org.github.io#305 for relevant confusion.

[benjagm]

Discussed in OCWM past 2024/02/26. We hope to improve metaschemas documentation as soon as #158 progress.