Add UI Building Blocks page: new/removed templates, mixins, widget types, addons

Author: DanderBot

docs/patches/12.1.0/index.md

@@ -45,11 +45,16 @@ Each entry says what an API **does** if it's new, or what's **different** if it
 - [Gameplay: PvP, Items, Spells, Specs](gameplay.md)
 - [Social, Battle.net & Discord](social.md)
 - [Housing](housing.md)
+- [UI Building Blocks](ui-blocks.md) — templates, mixins, widget types, new addons
 
 ---
 
 ## Caveats / what this does NOT cover
 
-- **Intra-struct field changes are invisible to a surface diff.** The private-aura `showCountdownFrame`→`showCooldownFrame` rename was caught only by comparing the files directly. Treat struct-field renames as a manual-check item.
-- **Engine/runtime behaviour is not covered** — taint-rule changes, secret-value handling, `error → nil` behaviour, and re-opened or newly-restricted APIs do not show up in a source diff. Cross-check against the official PTR patch notes and the WoW Dev Discord `#api-changes` channel.
-- Reflects PTR build **12.1.0.68209** and is subject to change before 12.1 goes live.
+Coverage is being filled in stages. Currently in: the documented API surface (functions, events, structs, enums) and the **UI building blocks** (templates, mixins, widget types, addons). Still being added / out of scope:
+
+- **Field-level changes inside existing structs & enums** aren't yet enumerated automatically. The private-aura `showCountdownFrame`→`showCooldownFrame` rename was caught by manual inspection; a field-level pass over the API docs is planned to make these exhaustive.
+- **Global functions & constants defined in Blizzard's addons** (not in the generated docs) aren't catalogued yet — planned.
+- **Blizzard's internal implementation changes** (function-body rewrites that don't change a public signature, template, or global) are intentionally excluded — they can't affect addons. Use the raw source diff for those.
+- **Engine/runtime behaviour** — taint rules, secret-value handling, `error → nil`, re-opened/restricted APIs — isn't visible to a source diff. Cross-check the official PTR patch notes and the WoW Dev Discord `#api-changes` channel.
+- Reflects PTR build **12.1.0.68209**; subject to change before 12.1 goes live.