Mark remote sampling API as stable

[yurishkuro]

Which problem is this PR solving?

• Related to Invalid expectations of JSON format for Jaeger remote sampling strategies open-telemetry/opentelemetry-specification#3126

Short description of the changes

• Describe remote sampling API as official / stable, with examples.
• Mark / endpoint on the agent as deprecated / not recommended.
• Replace references to "Jaeger clients" with general "SDKs" where appropriate.
• Add more warnings that Jaeger clients are deprecated.
• Mention OTLP/JSON format as accepted.
• Fix the positioning of the Copy button in the code snippets.

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