Docs: Add note about Favicon Caching in Vercel

[columk1]

What?

This PR adds a note to inform users of Vercel's build cache, which makes it difficult to replace favicon files once deployed.

Why?

There are many posts about issues replacing favicons on Stack Overflow, Reddit, Github etc. Many solutions involve renaming favicon.ico in the app folder to icon.ico, which works well but it's not clear why from what is said in the docs.

The docs don't mention that Vercel caches the favicon in the build cache and will not replace the file unless the build cache is invalidated or a different file is pointed to in the metadata.

Using any of the cache invalidation options at the link below will allow a favicon file to be updated after it has been deployed.

https://vercel.com/docs/deployments/troubleshoot-a-build#managing-build-cache

[ijjk]

Allow CI Workflow Run

• approve CI run for commit: 1a068a3

Note: this should only be enabled once the PR is ready to go and can only be enabled by a maintainer

[leerob]

Is there a reproduction for this issue? So you are saying the file itself has changed, but it still cached?

[columk1]

Is there a reproduction for this issue? So you are saying the file itself has changed, but it still cached?

Yes, I replicated it here using create-next-app:
https://github.com/columk1/test-next-favicon

When favicon.ico is deleted the Vercel deployment correctly has no favicon, but when a new favicon.ico is added, the old Vercel favicon re-appears.

There is more discussion here:
https://github.com/orgs/vercel/discussions/1808#discussioncomment-9426338