docs(examples): use vercel integration in cms-sanity

[stipsan]

Feature

• [x] Documentation added

Documentation / Examples

• [x] The examples guidelines are followed from our contributing doc

This PR updates the cms-sanity example to:

• Deploy to Vercel with the Sanity Vercel Integration by default.
• You can still opt-out of using the Sanity Vercel Integration, clicking You can also set up manually gives you all the information you need.
• The blog itself is updated so it's much more resilient to missing data, and is setup to set revalidate: 60 until you have a On-demand Revalidation webhook setup.
• Preview Mode is now enabled on the frontpage as well.
• The Sanity client setup, and webhook validation, are updated to follow our current best practices.