Update ADK doc according to issue #1292 - 3 - token compaction

[adk-bot]

3. Document the new Token-based Compaction feature in Context Compaction.

Doc file: docs/context/compaction.md

Current state:

The documentation only covers sliding window compaction using compaction_interval and overlap_size.

Proposed Change:

Add a new section 'Token-based Compaction'.
Explain that you can alternatively configure compaction based on the estimated token count of the session history.

• token_threshold: The token count that triggers compaction.
• event_retention_size: The number of recent raw events to keep uncompressed.
  Mention that if token-based compaction is configured and triggered, it takes precedence over sliding window compaction for that turn.
  Update the EventsCompactionConfig example to show these new parameters.

Reasoning:
The new token-based compaction feature allows for more dynamic context management based on actual token usage rather than just event count. Users need to know how to configure this.

Reference: src/google/adk/apps/compaction.py

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