[Support Feedback] Add TTFB, caching, load balancing, and Argo troubleshooting (Speed/Cache)

[dmmulroy]

Summary

Adds and expands documentation for common speed, caching, load balancing, and Argo support topics, based on an audit of active support macros.

Changes

• TTFB / reverse proxy: Add FAQ explaining why TTFB may appear higher with Cloudflare and how to reduce it
• Observatory / speed testing: Add FAQ with diagnostic steps and what info to provide when requesting support
• CF-Cache-Status troubleshooting: Add per-status troubleshooting guidance (missing header, persistent DYNAMIC/MISS, unexpected BYPASS)
• Session persistence: New troubleshooting page covering cookie stripping by cache, IP-based session breakage, and header modification by Workers/rules
• Large file serving: Expand the video/large file policy page with R2 and Images as alternatives alongside Stream
• Load balancing: Add diagnostic checklist and API commands for gathering LB config when troubleshooting
• Argo billing: Expand billing section with per-domain charges, billing cycle lag, cache hit inclusion, and usage monitoring

Context

These changes are driven by recurring support cases. If you want to see the underlying support data (macro frequency, case volume by topic), reach out to @dmmulroy internally.