Align README docs structure with current IA

[crazywriter1]

Summary

• Update README contributor guidance to the live tree: get-started, build-on-base, base-chain, specifications, sdks, Upgrades.
• Stop pointing contributors at removed folders (learn, cookbook, mini-apps, onchainkit, apps, top-level base-account/base-app) and leftover docs/agents.

Test plan

• README lists folders that exist under docs/ on master
• License badge URL is unchanged
• No other files in the diff

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1