docs: Add some helpful notes for Incremental Static Regeneration

[leoj3n]

Hopefully this is helpful, explaining some of the steps one would have to go through for Vercel ISR in more detail.

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[leoj3n]

A number of additional things are touched on in the comments of the referenced issues (seen above).

At any rate, I think these ISR docs could be more helpful overall, to better explain the functionality and limitations/edge cases users are likely to encounter.

As long as there is no dedication function for doing so, I think it would also be good to show a generic example of revalidating a given route (which most seem to do via fetch with the special header set). Something like:

• https://github.com/jdgamble555/isr-test/blob/master/src/routes/revalidate/%2Bpage.server.ts
• https://github.com/binsarjr/apiliburnasional/blob/7a10a92a339d9ccaaba71f5f6e1be28aa879fcaf/src/routes/api/revalidate/%2Bserver.ts