ci: validate devhub build on docs changes

[pkosiec]

Summary

• Add a devhub-validation CI job that runs when docs/** changes
• Clones devhub, syncs appkit docs from the PR branch, and runs the devhub build
• Catches docs changes that would break the devhub site before they land on main
• Supports forks via APPKIT_REMOTE env var override

Depends on databricks/devhub#58 for APPKIT_BRANCH / APPKIT_REMOTE env var support in the devhub sync script.

⚠️ TODO BEFORE MERGE: Switch devhub clone from pkosiec/appkit-docs-sync branch to main once databricks/devhub#58 merges.

Test plan

• CI runs the DevHub Build Validation job — successful run
• Job clones devhub, syncs docs from this branch, and builds successfully (3m58s)