[WIP] Preparation for v3.0.0 release: Upgrade guide & Changelog#5589

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rwgk wants to merge 9 commits intopybind:masterfrom

rwgk:prepv300/review

Commits on Mar 30, 2025

Squashed prepv300/manuscript — 30b9c268aeb98308ea42aaccfd5fe454e173c6fc — 2025-03-30 14:56:03 -0700 [skip ci]
rwgk
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Commits on Mar 31, 2025

Commits on Apr 7, 2025

[skip ci] Explain type_caster_enum_type_enabled, copyable_holder_caster_shared_ptr_with_smart_holder_support_enabled, move_only_holder_caster_unique_ptr_with_smart_holder_support_enabled in Upgrade…
rwgk
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[skip ci] Add a small section for py::bind_vector, py::bind_map & py::smart_holder
rwgk
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[skip ci] Fix tiny oversight: Reference back to the current release v2.13 (not v2.12)
rwgk
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Remove sentence: Using self._pybind11_conduit_v1_() ... should keep extension compatibility.
rwgk
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Changelog: combine #4953 and #5439
rwgk
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[skip ci] Trivial whitespace/formatting fixes/enhancements.
rwgk
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