Merge pull request #33437 from backstage/techdocs-bui-header-migration

feat(techdocs): migrate alpha plugin pages to BUI header system

.changeset/easy-pens-judge.md

@@ -0,0 +1,7 @@
+---
+'@backstage/ui': patch
+---
+
+Made `SearchAutocomplete` background-aware. The input now adapts its background color based on its parent container's background level.
+
+**Affected components:** SearchAutocomplete

.changeset/fuzzy-mangos-design.md

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+---
+'@backstage/plugin-techdocs': patch
+---
+
+Migrated the TechDocs alpha plugin pages to use BUI components. The index page and reader page now use BUI `Header` and `Container` instead of legacy `Page`/`Content`/`ContentHeader` wrappers. Added a `SupportButton` as a plugin header action. Changed plugin title to "Documentation" and icon to `RiArticleLine`.

.changeset/happy-masks-allow.md

@@ -0,0 +1,7 @@
+---
+'@backstage/ui': patch
+---
+
+Added `aria-hidden` to the `PluginHeader` icon to prevent screen readers from announcing decorative plugin icons.
+
+**Affected components:** PluginHeader