Update GitHub Pages workflow example

[bencampion]

I've updated the GitHub Pages workflow example to use actions for the actual deployment and to set --pathprefix.

I've also change how the caching works as I don't think the previous example would have worked correctly for most use cases. You can't change the contents of an existing cache so if you keep on reusing the same cache key then you'll eventually end up with a saved .cache folder that only contains stale entries.

I think it would be more useful to restore the cache saved from the last run so I've changed the cache action to use a key based on the current timestamp so it will always create a new cache on each run. This means there will never be a direct cache hit on subsequent runs, but I've also set restore-keys so it will restore the last saved cache (from the same branch) on a miss.

I'm quite new to GitHub actions so maybe there is a more elegant way to achieve this behaviour!

[vercel]

@bencampion is attempting to deploy a commit to the 11ty Team on Vercel.

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