[codex] migrate MkDocs Material build

[Frichetten]

Summary

• Replace the deleted mkdocs-material-insiders install in deploy CI with the public mkdocs-material[imaging]>=9.7.0 package.
• Rename the deploy-specific MkDocs config from mkdocs.insiders.yml to mkdocs.deploy.yml.
• Update local deploy Dockerfile and contribution docs to remove stale Insiders/subscription assumptions.

Root cause

The deploy workflow was still cloning squidfunk/mkdocs-material-insiders, which now returns Repository not found after Material for MkDocs made Insiders features available in the public package.

Validation

• mkdocs build --site-dir /private/tmp/htc-site-default
• mkdocs build --config-file mkdocs.deploy.yml --site-dir /private/tmp/htc-site-deploy

The deploy-config local build completed with public mkdocs-material 9.7.6. The unauthenticated local run hit the existing git-committers GitHub API rate limit, but CI now provides GH_TOKEN via github.token.