re #15 docs: fill in package index

[tonydspaniard]

Summary

Fills in docs/README.md to replace the four _Coming soon: …_ placeholders introduced in #16 with real bulleted entries for all 15 sibling-package docs (PRs #24–#38).

Taxonomy

Lightly reorganized into four topical sections — each entry is a one-line punch describing what the package does:

• HTTP stack — Http, Cookie, Session, Sanitation, Validation
• Application core — Container, Configuration, Happen, Courier, Middleware
• Data & types — Data, Structure, Common
• Infrastructure — Cache, Filesystem, Security

Three of those slot differently than the original placeholders suggested (Sanitation and Validation moved from "Data & domain" to "HTTP stack" — they exist to clean and validate inbound HTTP input; Http moved from "Infrastructure" to "HTTP stack" where it belongs). Easy to reshuffle in review if you want a different grouping.

Merge order

This PR's links assume PRs #24–#38 will be merged first — each link points to a sibling ./packages/<pkg>.md that lives in one of those PRs. If you merge this one first, all 15 links will 404 until the package PRs land. Suggested order: merge any subset of #24–#38 (in any order), then merge this one last.

Test plan

• Sibling package PRs merge (any order) before this one
• Render docs/README.md on GitHub, check all 15 links resolve
• Pure additive — no code touched