docs(logging): document log record enrichment

[popsiclelmlm]

Summary

Fixes #2642.

Documents the logging instrumentation behavior in the package README, including:

• exporting Python log records as OpenTelemetry logs
• opt-in trace context injection / log correlation
• injected logging.LogRecord attributes
• relevant environment variables and matching LoggingInstrumentor().instrument(...) arguments

Reproduction

The logging instrumentation README currently only covers installation and references. It does not explain the log record enrichment behavior requested in #2642.

Root cause

The enrichment and configuration details are implemented in the instrumentation, but the package README did not describe the behavior or fields added to log records.

Changes

• Added usage guidance for LoggingInstrumentor.
• Added a trace context injection section listing otelSpanID, otelTraceID, otelServiceName, and otelTraceSampled.
• Added a configuration section covering the logging-related environment variables and kwargs.
• Renamed the generated docs reference link text to avoid a duplicate implicit RST target.

Tests

• git diff --check
• /tmp/otel-rstcheck-venv/bin/rstcheck instrumentation/opentelemetry-instrumentation-logging/README.rst

Screenshots/Logs

N/A. Documentation-only change.