docs: document exception hierarchy

[Boulea7]

What was wrong?

The exception hierarchy and JSON-RPC error fallback behavior were only described in migration notes, making it difficult for users to choose the right exception boundary.

Related to Issue #3442
Closes #3442

How was it fixed?

• Add a focused guide for the Web3Exception hierarchy and common branches.
• Explain method-specific error formatters, Web3RPCError specialization, and message-based fallback behavior.
• Link the guide from the documentation table of contents.
• Add a documentation newsfragment.

Validation

• make check-docs (HTML build, 205 doctests, newsfragment validation)
• Strict targeted Sphinx build with nitpicky warnings treated as errors
• Targeted pre-commit checks, including the repository-wide mypy hook

Todo:

• Clean up commit history
• Add or update documentation related to these changes
• Add entry to the release notes

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