Improve docs information architecture

[adnaan]

Summary

• add a getting-started mental model page for templates, controllers, state, transports, and pub/sub
• reorganize sidebar navigation into Start, Build, Patterns, Apps, Deploy, Reference, CLI, and Client sections
• add homepage comparison/fit guidance and consistent transport wording
• add at-a-glance tables to long reference pages and descriptions to high-traffic docs
• add GOWORK=off test targets, including local e2e, plus README guidance
• document client CDN versioning guidance

Paired tinkerdown PR

• Depends on/was previewed with Improve docs chrome navigation metadata tinkerdown#283 for source metadata, server-rendered search, debug meta, and long-page H3 TOC behavior.

Validation

• go run /home/adnaan/code/livetemplate/tinkerdown/cmd/tinkerdown validate content/ -> 56 valid, 0 errors
• make test-unit
• make test-examples
• GOWORK=off E2E_BASE_URL=http://127.0.0.1:8084 go test ./e2e/... -count=1

Preview

• Local preview was run at http://100.123.67.113:8084 with recipes served on :9091.