Update deploy.yml and added dependabot.yml

[harshdoesdev]

• Install fastn
• Checkout to the main branch
• Run fastn build with offline flag cause we don't have to download any package file again since they are already checked in
• publish the build artifacts to vh_page branch

[amitu]

Add another action also, which will run on each PR comment, and verify that fastn build works, without updating vh_pages, or better yet, modify this file to only update vh_pages if a commit happens on main branch, and for pull requests it simply reports if the fastn build succeeded or not.