Add missing docstrings for Item/MediaReference, fix RationalTime math formatting

[hikmetba-bit]

Summary

Fixes #2042 (partially — targets the three concrete examples named in the issue: Item, MediaReference, RationalTime).

• Item and MediaReference had no class-level docstring in their pybind bindings (py::class_<Item, ...>(m, "Item", py::dynamic_attr()) with no doc argument), so https://opentimelineio.readthedocs.io/en/stable/tutorials/otio-serialized-schema.html#item-1 and #mediareference-1 showed None. Added docstrings following the existing style used by sibling classes (Composition, Clip), cross-referencing their concrete subclasses.
• RationalTime's docstring used a Sphinx :math: role referencing an undefined rt variable (:math:\rt.value/rt.rate`). This role is never actually rendered as math — the same docstring is also dumped verbatim into a plain code fence by autogen_serialized_datamodel.py` for the schema doc page — so it just showed up as confusing literal text. Replaced it with plain double-backtick formatting and expanded the description slightly, matching the issue's "too short" note.

docs/tutorials/otio-serialized-schema.md is regenerated by hand to match, since test_serialized_schema.py asserts the checked-in file matches what autogen_serialized_datamodel.py produces from the live docstrings.

Test plan

• Could not build the C++ extension in this environment (no CMake/MSVC toolchain available), so test_serialized_schema.py was not run directly.
• Manually reproduced the generator's exact text-processing (inspect.cleandoc + the textwrap.wrap(width=100, replace_whitespace=False, drop_whitespace=False, break_long_words=False) pass from autogen_serialized_datamodel.py) against the new docstrings in a plain Python script to confirm the wrapped output matches byte-for-byte what was written into otio-serialized-schema.md.
• Cross-checked that every attribute/class referenced in the new docstrings (source_range, available_range, available_image_bounds, value, rate, ExternalReference, GeneratorReference, ImageSequenceReference, MissingReference) actually exists as a bound property/class in the same bindings files.
• Did not touch otio-serialized-schema-only-fields.md, since that file omits docstrings entirely and is unaffected.

🤖 Generated with Claude Code

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.48%. Comparing base (a639246) to head (a2eecf9).

Files with missing lines	Patch %	Lines
...entimelineio-bindings/otio_serializableObjects.cpp	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2051   +/-   ##
=======================================
  Coverage   83.48%   83.48%           
=======================================
  Files         182      182           
  Lines       13517    13517           
  Branches     1254     1254           
=======================================
  Hits        11284    11284           
  Misses       2060     2060           
  Partials      173      173           

Flag	Coverage Δ
py-unittests	83.48% <0.00%> (ø)

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Files with missing lines	Coverage Δ
...lineio/opentime-bindings/opentime_rationalTime.cpp	87.03% <ø> (ø)
...entimelineio-bindings/otio_serializableObjects.cpp	84.46% <0.00%> (ø)

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