README overhaul for bp-7.0

[ghbarker]

Rewrites the README so the documentation is ready ahead of the bp-7.0 patch.

What changed

• Complete settings reference — every selection in the BluePilot menu (comma 3X, comma 4, and sunnylink remote), organized by menu section with types, options, value ranges, device markers, and gating notes. Verified against bp-dev.
• BluePilot vs sunnypilot attribution — a "Where features come from" table up front, a BluePilot-only settings reference, and a separate upstream sunnypilot section (MADS, NNLC, DEC, SLA, ICBM, SCC-M/V, Model Manager, sunnylink) so users can tell which project owns which feature.
• New sections: Vehicle Compatibility (LCA vs LKA explainer, confirmed TRON status table, known-working vehicles), expanded Installation (Q3/Q4 harness rules, per-vehicle install videos, cable spec, Lightning Coupler, upgrade notes), Recommended Settings (angle and curvature model guidance), Tuning Guide with video links, Troubleshooting & FAQ, How to Ask for Help (log upload walkthrough), and Version History.
• Angle-first ordering throughout — angle mode leads the selector description, tuning guide, recommended settings, and FAQ.
• Housekeeping — all external links checked (replaced the dead discord.sunnypilot.com invite with discord.gg/sunnypilot), stale pre-7.0 instructions pruned, duplicate headings and invalid markup fixed.

Notes for reviewers

• Settings tables were enumerated from the bp-dev settings code, not from any open PR branch, so they reflect what ships today; if the 7.0 patch adds new lateral controls, the Angle Tuning table will need those rows.
• BP_CHANGES.json has no 7.0 entry yet; the Version History section links it as covering bp-4.0 through 6.x.