docs: align 2.2 encoding docs and nested add-column notes

[Xuanwo]

Cover more 2.2 changes in our docs to get users perpared.

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Parts of this PR were drafted with assistance from Codex (with gpt-5.3) and fully reviewed and edited by me. I take full responsibility for all changes.

[github-actions]

Code Review

Review Summary: Documentation-only PR - LGTM ✅

This PR improves documentation clarity for Lance 2.2 encoding features. I verified the key claims against the codebase:

1. RLE BlockCompressor: Confirmed - RleEncoder implements BlockCompressor trait (rust/lance-encoding/src/encodings/physical/rle.rs:393)

2. General BlockCompressor: Confirmed - CompressedBufferEncoder implements BlockCompressor (rust/lance-encoding/src/encodings/physical/block.rs:627)

3. VariablePackedStruct: Confirmed - defined in protos/encodings_v2_1.proto:425 with field number 13

No P0/P1 issues found. The documentation changes are accurate and improve clarity for:

• Miniblock metadata wording for final chunk decoding
• PackedStruct vs VariablePackedStruct distinction by version
• Compression capability matrix updates for 2.2

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Reviewed via automated analysis