Add guidance around use of title and description keywords

[jack-berg]

Related to #142, which references that "Type title will not change."

I still agree with this, but don't think we should define type titles using the title keyword. Instead, the title should be sourced from the key values withing $defs. For example, given the following from meter_provider.json, the periodic metric reader's title is PeriodicMetricReader. See PR content for reasoning.

   ...
    "$defs": {
        "PeriodicMetricReader": {
            "type": "object",
            "additionalProperties": false,
            "properties": {
                "interval": {
                    "type": ["integer", "null"],
                    "minimum": 0
                },
                "timeout": {
                    "type": ["integer", "null"],
                    "minimum": 0
                },
                "exporter": {
                    "$ref": "#/$defs/PushMetricExporter"
                },
                "producers": {
                    "type": "array",
                    "items": {
                        "$ref": "#/$defs/MetricProducer"
                    }
                }
            },
            "required": [
                "exporter"
            ],
            "title": "PeriodicMetricReader"
        },
   ...

[jack-berg]

Notably, removing title in top level schema like this, which collect types for re-use but which aren't referenced anywhere in the schema directly tidies up the result of code generation.

Previously, the opentelemetry-java implementation generated a CommonModel type. It wasn't used anywhere, but it still was generated. With this change, its goes away as it should. I could probably find some way to explicitly omit generation for these kind of types, but it is a nice side affect of what seems like a sensible change for other reasons.

[marcalff]

LGTM.