docs: header bar, debug-level chips, column view setting and 1.21.0 changelog links

[lukecotter]

Fills the documentation gaps for features that shipped in 1.21.0 and repairs the changelog reference block.

CHANGELOG

• Adds the missing [#828]: link definition — it was referenced twice with no target.
• Links the database table columns, data grids and database usage bars bullets to #873.
• Fixes a typo ("no longer flag" → "flagged").

Bullets with no real issue behind them are left unlinked rather than given invented numbers.

Docs

• New features/header.mdx — log problems chip, notifications bell, Inspector toggle, the ••• overflow menu, card behaviour, the unsupported-log-event report action, and the read-only debug-level chips with their +N popover. Added to the sidebar ahead of Timeline.
• features/timeline.mdx — exception hairline markers, "N exceptions" cluster collapsing, and the Throws tooltip row.
• settings.mdx — documents lana.callTree.columnView (default General; Time, Governor Limits, Database, Memory).
• features/calltree.mdx — cross-reference to that setting.

README

• New ## 🚦 Header section plus a Key Features bullet.
• Fit-to-content column widths noted under Call Tree.

Root README.md only; the lana/ copy is generated at build time.

No screenshots — all assets still point at 1_20. The 1_21 capture pass is separate.