Some documentation improvements

Please proof-read this very carefully, documentation is important, and I don't
know much about daybed yet, so I could very possibly be wrong about some of
those modifications.

Also, if you have a better idea than replacing all the `v1` everywhere in the
URLs on the next version (like using a variable or something), please let me
know (i've checked ext_link and rst_epilog, but none seem to do the job... or
be usable in code blocks).
Tests about model: https://github.com/spiral-project/daybed/blob/master/daybed/tests/test_schemas_object.py#L39-L44 - https://github.com/spiral-project/daybed/blob/master/daybed/tests/test_schemas_object.py#L52-L57 - https://github.com/spiral-project/daybed/blob/master/daybed/tests/test_schemas_object.py#L107-L144