Split Overview into the two specific use cases (resubmission)

[gregsdennis]

What kind of change does this PR introduce?

Issue & Discussion References

Replaces #1370

Summary

[!NOTE] This is a resubmission of @awwright's #1370. Please refer there for extensive conversation. The below text is also from that PR.

This is an alternative to https://github.com/json-schema-org/json-schema-spec/pull/1244, and like that issue, this one should be incorporated prior to https://github.com/json-schema-org/json-schema-spec/issues/1365. This PR makes a clearer distinction between validation uses and annotation uses that 1365 will draw on (it relies on classifying which use each keyword is being used for, since validation use is different from annotation use).

This PR moves vocabulary-related discussion deeper into the specification, and replaces Overview with a specific description of what a JSON Schema can do, especially in terms of its output.

Once its capabilities are described, the rest of the specification can describe how you write a schema that can do these things.

This replaces the Overview section, which is a little bit redundant with the above Abstract, then Introduction, sections.

Does this PR introduce a breaking change?

No.