Add v3 to v4 migration guide

[jtdub]

Summary

Adds docs/user/migrating-from-v3.md (nav: User Guide → Migrating from v3, right after Getting Started) — the missing piece for existing 3.x users, since v4 renames or removes nearly every entry point.

Contents:

• Quick-reference rename tables: constructors (get_hconfig → HConfig.from_text, etc.), methods (config_to_get_to → remediation(), dump_simple → to_lines(), ...), and utilities (load_hconfig_v2_options → load_driver_rules, removed v2 mappers).
• Negation rule migration with before/after code for all three old models → NegationRule + NegationStrategy, and a note that load_driver_rules() still accepts the old dict keys.
• Typed exceptions table (ValueError/TypeError → HierConfigError subclasses) for updating except clauses.
• Config-view transition: isinstance(view, InterfaceVlanViewMixin) replacing NotImplementedError catching.
• Behavior changes to review: future() negation resolution (future() negation edge cases: value-carrying negation on idempotent lines leaves a literal 'no ...' line; bare shorthand negation never matches; empty parents not pruned #269), structured-input rejection, registry-based custom driver wiring.
• New-in-v4 pointers for features worth adopting during the upgrade.

Test plan

• Every import and API name in the guide executed against the package (InterfaceVlanViewMixin, HierConfigError, NegationRule/NegationStrategy, load_driver_rules/load_tag_rules, ...).
• poetry run mkdocs build --strict — zero warnings.
• poetry run ./scripts/build.py lint-and-test — "No issues found".

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