docs: replace placeholder product documentation

[ruru-m07]

Summary

• replace placeholder Fumadocs pages with 21 implementation-grounded product guides
• reorganize navigation around setup, changes, history, branches, advanced workflows, and reference
• remove unshipped or planned pages from navigation, search, and LLM exports
• add spacing above the bottom page navigator and replace broken header links with Roadmap
• document current beta limitations, including clone authentication and interactive rebase behavior

Validation

• bun --cwd=apps/web run check-types
• bun --cwd=apps/web run cf-build
• make verify
• browser smoke test on /docs and /docs/branching/upstream-tracking

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