docs: add Model API reference

[GoparapukethaN]

Closes #3369

Adds bentoml.Model to the Bento/model API reference page so SDK methods such as export and import_from are discoverable from the generated reference docs.

Verification:

• Confirmed bentoml.Model imports from an editable local BentoML install and exposes export, import_from, load_model, and with_options.
• Ran git diff --check.
• Ran a Sphinx dummy build with a temp config that skips the local spelling extension because this machine does not have the enchant C library installed. The build completed with existing docs warnings.