Add missing docstrings for Item, MediaReference, RationalTime

[hikmetba-bit]

Summary

Addresses the three concrete examples named in #2042:

• Item had no class docstring at all.
• MediaReference had no class docstring at all.
• RationalTime's docstring used a Sphinx :math: role (:math:\rt.value/rt.rate`) that renders as raw, confusing text outside of the actual Sphinx build — e.g. in the autogenerated docs/tutorials/otio-serialized-schema.md, or in plain help()` output.

While in there, I also documented Item.source_range/color/markers/effects, MediaReference.available_range/available_image_bounds/is_missing_reference, and SerializableObjectWithMetadata.name/metadata. That last one is inherited by essentially every schema class, so it fixes the empty name/metadata parameter descriptions across the whole generated schema doc, not just on Item/MediaReference.

This is #2042's three named examples plus one high-leverage base class — not the full documentation checklist the issue asks for, which is a much bigger, open-ended audit better done incrementally.

Important note for the merger

docs/tutorials/otio-serialized-schema.md is generated by make doc-model-update, which requires building the native _otio extension. I don't have a C++ toolchain in this sandbox, so I could not run it, and deliberately left that generated file untouched rather than hand-edit it (the name/metadata change alone would touch dozens of unrelated lines across the whole file, which is exactly the kind of thing that file's generator, not a human, should produce). The project's own test suite should flag that it's stale — running make doc-model-update before merge will refresh it correctly.

Test plan

• Could not build/run the test suite locally (no C++ toolchain available in this environment).
• Verified by manual review: cross-checked every new :class:~X`` reference against the actual bound class names in the same file, confirmed the def_property/`def_property_readonly` docstring-argument pattern against existing working examples in this same file (e.g. `Item.enabled`), and traced through `autogen_serialized_datamodel.py` to confirm exactly which docstrings feed the generated schema doc (via `inspect.getdoc`) before deciding what to change.

🤖 Generated with Claude Code

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[hikmetba-bit]

Closing as a duplicate of #2051, which I opened concurrently in a separate session and just consolidated with the extra property-level docstrings from this PR (Item.source_range/color/markers/effects/available_image_bounds, MediaReference.available_range/available_image_bounds/is_missing_reference, RationalTime.value/rate). Apologies for the noise — #2051 now has everything from both.