docs: add Base Sepolia quickstart example

[GomezAdalyn]

Summary

• Add a quickstart example for Base Sepolia
• Provide a ready-to-copy withdrawer command for L2 withdrawals

Rationale

• Makes it easier for users to run withdrawer against Base Sepolia without guessing flags

[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

[GomezAdalyn]

Happy to adjust the wording or flags if you prefer a different Base Sepolia RPC example or format