docs: add new arbos50 release

[zk-Lumi]

Description

This pull request adds the formal documentation and guidelines for arbos50: Dia. This PR also updates the ArbOS upgrade guide to use ArbOS 50 as the example.

DO NOT MERGE UNTIL:

• Nitro v3.9.0 release has been cut
• Docker image for Nitro v3.9.0 & WASM module root sections are updated in docs pages
• Audit report from Trail of Bits is publicly published
• Tally proposal has passed? (we may publish this doc as before Tally proposal, and submit another PR with link for proposal afterwards)

Document type

• Gentle introduction
• Quickstart
• How-to
• Concept
• FAQ
• Troubleshooting
• Reference
• Third-party content
• Not applicable

Checklist

• I have read the CONTRIBUTE.md guidelines
• My changes follow the style conventions outlined in CONTRIBUTE.md
• I have used sentence-case for titles and headers
• I have used descriptive link text (not "here" or "this")
• I have separated procedural from conceptual content where appropriate
• I have tested my changes locally with yarn start or yarn build
• My code follows the existing code style and conventions
• I have added/updated frontmatter for new documents
• I have checked for broken links
• I have verified that my changes don't break the build
• Third-party docs only: Do you agree to the third-party content policy outlined within CONTRIBUTE.md?
  • Yes
  • Not applicable

Additional Notes

[vercel]

@zk-Lumi is attempting to deploy a commit to the Offchain Labs - AF Billed Team on Vercel.

A member of the Team first needs to authorize it.

[CLAassistant]

All committers have signed the CLA.