Add support levels to gateway API compatibility doc

[kate-osborn]

There are three different support levels in the Gateway API:

• Core features will be portable and we expect that there is a reasonable roadmap for ALL implementations towards support of APIs in this category.
• Extended features are those that are portable but not universally supported across implementations. Those implementations that support the feature will have the same behavior and semantics. It is expected that some number of roadmap features will eventually migrate into the Core. Extended features will be part of the API types and schema.
• Implementation-specific features are those that are not portable and are vendor-specific. Implementation-specific features will not have API types and schema except via generic extension points.

Update the compatibility doc to include the support levels for each feature.

[mpstefan]

We discussed today about updating the overall conformance level of the individual api objects to something like "core" or "v1beta1 core" and removing any verbage about fields being "partially supported". Once we are at core for a feature, we no longer need a notice about partial support as we will have the support we define for the api object.