Add Sphinx docs for opentelemetry.sdk.util package

[OmYarewar]

Description

Ran into this while looking through the SDK docs — the opentelemetry.sdk.util package has several public utilities (BoundedList, BoundedDict, ns_to_iso_str, get_dict_as_key) but none of them show up in the generated Sphinx documentation. Only the instrumentation submodule had a docs page, and it was sitting under the trace toctree which didn't feel right since it's actually part of sdk.util, not sdk.trace.

Changes:

• Added docs/sdk/util.rst with an automodule directive for opentelemetry.sdk.util so BoundedList, BoundedDict, ns_to_iso_str, and get_dict_as_key all get documented
• Moved the util.instrumentation toctree entry from trace.rst to util.rst where it actually belongs
• Added util to the SDK docs index

Fixes #2573

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Verified RST structure matches the existing pattern used by metrics.rst, trace.rst, etc.
• Confirmed util.rst follows the same automodule + toctree pattern as other SDK doc pages
• Need to run tox -e docs locally to verify the generated HTML renders correctly (don't have the full tox setup right now)

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

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