[F-3] Navigate a long article by its table of contents

[liangricky7]

Blocked by: F-2 (#27) · Blocks: —

(Was sprint 1 ticket 6.) Jump straight to the section you need instead of scrolling for it.

• With three or more headings, a table of contents appears in the left rail, built from the toc entries T1-3 ([T1-3] Read an article in one request #6) returns. Nobody maintains it by hand
• Two levels deep: top-level entries in accent, second-level entries in ink-soft and indented
• Below three headings there is no table of contents at all. The rail is absent, not empty
• Clicking an entry, or activating it from the keyboard, scrolls to that section
• Link anchors use the section's id, not its heading text, so renaming a heading never breaks a link to it

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Source: docs/frontend-tickets.md