docs: add specification page for standard table config keys

[Copilot]

Manifest.config reserves the lance. prefix for the format itself, but there was no single reference listing which keys exist, what they mean, or what values they accept — implementers had to grep the Rust source to find them.

Changes

• New spec page: docs/src/format/table/table_config.md documents every known lance.-prefixed config key found in the codebase, with type/format and description, grouped by feature:
  • lance.auto_cleanup.* (interval, older_than, retain_versions, referenced_branch, delete_rate_limit)
  • lance.compaction.* (all 15 keys accepted by CompactionOptions::apply_dataset_config)
• Navigation: registered the page in docs/src/format/table/.pages.
• Cross-links: linked from the Manifest section and "Related Specifications" in docs/src/format/table/index.md, and from the auto-cleanup section in docs/src/guide/read_and_write.md.

Example

| Key                                       | Type                  | Description                                          |
|--------------------------------------------|-----------------------|-------------------------------------------------------|
| `lance.auto_cleanup.interval`               | Non-negative integer  | Commits between automatic cleanup runs. `0`/unset disables it. |
| `lance.compaction.target_rows_per_fragment` | Non-negative integer  | Row-count threshold below which a fragment is a compaction candidate. |

• Fixes docs: create a spec page listing all standard table configurations #8672

[github-actions]

Important

This PR touches the Lance format specification.

Substantive changes to the format specification — the .proto definitions
and the spec docs under docs/src/format/ — require a PMC vote before merge.
Minor edits such as typo fixes, wording, or formatting are excluded; use your
judgment.

If this is a meaningful format change:

• Start a vote following the Lance community voting process.
  Format specification modifications need 3 binding +1 votes (excluding the
  proposer), held on GitHub Discussions, with a minimum voting period of 1 week.
• Once the vote passes, link the completed vote in this PR. It should not be
  merged until the vote is linked.

[github-actions]

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