Write specification doc describing schema format

[wjones127]

We should describe what data types are supported in schemas. They are represented as strings, so we should state what strings map to what Arrow types, for example.

Should also include an explanation of field ids.

Should go somewhere on https://lance.org/format/, either under file format or table format. The source for this is at:

https://github.com/lance-format/lance/tree/main/docs/src/format

[dik654]

I'm interested in working on this if it's still available. I'm not deeply familiar with the Lance codebase yet, but I'm happy to dig through the source to document the schema format. I may have some questions along the way — would it be alright if I gave this a try?

[westonpace]

You're welcome to work on this but the final result may be in flux as there are discussions such as #5864 and #5817 which may alter the set of supported types.