Document/formalize YAML schema for .mlem files

[aguschin]

... no need to include the schema in the README but probably somewhere for reference in the docs.

Also consider creating something like https://github.com/iterative/dvcyaml-schema

Originally posted by @jorgeorpinel in iterative/mlem#220 (comment)

[aguschin]

.mlem yaml schema is dependent on the type of the object. For model, data, deployment, etc, it will be a different schema. IIRC, this is the reason why we cannot easily create the schema as DVC can. We can take a look later, but this is not the priority rn.

[jorgeorpinel]

.mlem yaml schema is dependent on the type of the object

@aguschin we should start by listing the types of objects that can exist in .mlem files and general schema for them (model, data, deployment). This will help also improve the configuration docs (rel to -c option).