docs: mention and clarify meaning of ! in schemas

[hottwaj]

Please describe the issue

Current docs online about tables, schemas and data types do not mention that schema types may include an "!" prefix, e.g. a schema might look like this

{'not null float': '!float64',
 'nullable float': 'float64',}

see e.g. https://ibis-project.org/reference/schemas

Probably should be a separate ticket, but I'd also suggest that the use of ! to indicate "not null" could do with a re-think because I think its ambiguous depending on your background e.g. my expectation was that this was the correct interpretation

{'required float': 'float64',  # but ibis actually uses !float64 for this
 'not required float': '!float64',}  # ibis uses float64 for this

Thanks!

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[cpcloud]

Thanks, yep, that should be documented. Any interesting in making a PR to add some docs?

[ahaan1984]

Hi! Can I try working on this issue?

[cpcloud]

Hi! Can I try working on this issue?

Yes, of course!