🖋️ Scribe: Update exception module references

[fderuiter]

💡 What: Replaced outdated references to imednet.core.exceptions with imednet.errors across all .rst documentation files, updated the Sphinx Makefile target to correctly source src/imednet/errors, and fixed examples/custom_retry.py to import from the new module location.

🎯 Why: The codebase underwent an architectural refactor that centralized the exception hierarchy into src/imednet/errors. However, the documentation and example scripts were not updated, leading to ModuleNotFoundErrors for developers following the guides or trying to build the docs locally.

🧠 Cognitive Impact: Fixes broken onboarding paths and prevents immediate "first-run crashes." Developers copying the retry policies or offline validation examples from the documentation will no longer be hit by confusing import errors out-of-the-box.

📖 Preview:

# Before
from imednet.core.exceptions import RateLimitError, ServerError

# After
from imednet.errors import RateLimitError, ServerError

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PR created automatically by Jules for task 1473977327017387004 started by @fderuiter

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