[companies] add 9 new guides, refresh tips/questions from Discord, reorder sidebar

Mined the GreatFrontEnd premium Discord interview-experience archive to:
- Add fresh insider tips (2025-2026 entries) to 14 existing guides, trimmed to ~6 most-recent
- Add 9 new company guides: Coinbase, Discord, DoorDash, Mozilla, Netflix, OpenAI, Rippling, Robinhood, Squarespace
- Populate JavaScript / UI / System design question lists across all guides from specific Discord-reported questions, with GreatFrontEnd practice-question links verified against the live question catalog
- Reorder the company sidebar by prestige + interview volume (FAANG -> frontier AI -> big tech -> mid/private)
- Align the :::info banner: companies with a GreatFrontEnd guide now link to it directly; the rest fall back to the company directory

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>