Document staticPageGenerationTimeout config

[ijjk]

Adds note to documentation for the new static generation timeout.

Documentation / Examples

• Make sure the linting passes by running yarn lint

x-ref: #30110

[styfle]

Do we need to be more specific about "no activity occurs"? Could a response be streaming for 60 seconds and still timeout?

[ijjk]

@styfle re-worded the no activity a bit, a page has to finish getStaticProps/rendering needs to complete for some path in the worker for the timeout to not be hit