Add check to see whether an item can fit on a page at all

[diondokter]

This example:
https://github.com/cr1901/miniconf-sample/tree/main

The pages are only 16 bytes, so the given data doesn't fit. But it won't ever fit.
Currently the crate concludes that the storage must be full. However, with an extra check this error can be improved.

Might also be worth it to document the overheads a little bit like the page states and the item header size.