docs: Trim chat-history compaction blog section to be concise

[Copilot]

Motivation and Context

The initial docs/compaction.md draft read as full reference documentation rather than a blog section. A blog shared across multiple features needs to convey the concept quickly, not exhaustively document every API.

Description

Rewrote docs/compaction.md from ~200 lines down to ~55, keeping only what a reader needs to understand the feature:

• Concept — what compaction does and why it matters
• Key behavior — fires before every model call, including each function-calling loop iteration; tool-call/result pairs are never split
• Setup — single UseAIContextProviders snippet showing the registration pattern
• Strategies — one-line description per strategy with the CompactionTrigger concept illustrated inline
• Pipeline pattern — one clean PipelineCompactionStrategy example (gentle → aggressive) with a link to the full sample for detail

Removed: exhaustive trigger reference table, per-strategy code snippets, duplicate pipeline example, and strategy comparison table.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

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