docs: add popkins (sepolia + celestia mocha) and local OP Stack & Nitro devnet guides

[jcstein]

Summary

• adds guide on spinning up sepolia + mocha testnet op stack
• Add guide for spinning up a local OP Stack rollup with Celestia DA for testing
• add guide on running nitro local devnet
• Uses pop-deployer to generate pre-deployed bundles that run instantly via Docker
• adds guide on spinning up sepolia + mocha nitro

Test plan

• Verify the page renders correctly
• Check navigation from the OP section sidebar

[gemini-code-assist]

Summary of Changes

Hello @jcstein, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the documentation by introducing a new guide for setting up a local OP Stack development environment integrated with Celestia Data Availability. It also reorganizes existing Optimism-related documentation for better clarity and navigation, alongside standard dependency updates to maintain project health.

Highlights

• New Local Devnet Guide: A comprehensive guide has been added to set up a local OP Stack devnet with Celestia Data Availability, utilizing pop-deployer for pre-deployed bundles and Docker for execution.
• Documentation Structure Refinement: The documentation structure for Optimism-related content has been reorganized, including renaming a top-level navigation entry and creating a dedicated section for Optimism guides.
• Existing Guide Renamed and Relocated: An existing guide for deploying OP Stack rollups with Popkins, Popsigner, and Docker has been renamed for clarity and moved into the new Optimism documentation section.
• Dependency Updates: Various package dependencies and their integrity hashes in yarn.lock have been updated to ensure consistency and incorporate the latest versions.

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[github-actions]

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[gemini-code-assist]

Code Review

This pull request adds comprehensive documentation for setting up an OP Stack devnet, both locally and with external services like Popkins. The changes are well-structured and add significant value. However, the new popkins-popsigner guide contains several critical security issues related to hardcoded secrets and incorrect configuration instructions that must be addressed. I have also included some medium-severity suggestions to improve clarity and fix minor issues.