What does this change

• Fill out the Porter Architecture doc pages. Replace the TODO placeholder pages with actual info about porter's buildtime/runtime.
• Document porter's security features. Add a concept page that explains porter's security features.
• Add more information to the custom dockerfile page about the non-root user.

https://deploy-preview-2378--porter.netlify.app/architecture/
https://deploy-preview-2378--porter.netlify.app/architecture-buildtime/
https://deploy-preview-2378--porter.netlify.app/architecture-runtime/

https://deploy-preview-2378--porter.netlify.app (scroll down to the block that says security features)
https://deploy-preview-2378--porter.netlify.app/security-features/
https://deploy-preview-2378--porter.netlify.app/bundle/custom-dockerfile/#bundles-do-not-run-as-root (documented running as a non-root user)

What issue does it fix

Closes #1944
Closes #151

Notes for the reviewer

N/A

Checklist

• Did you write tests?
• Did you write documentation?
• Did you change porter.yaml or a storage document record? Update the corresponding schema file.
• If this is your first pull request, please add your name to the bottom of our Contributors list. Thank you for making Porter better! 🙇‍♀️

Reviewer Checklist

• Comment with /azp run test-porter-release if a magefile or build script was modified
• Comment with /azp run porter-integration if it's a non-trivial PR