changesets: add missing changesets for BUI header for NFS

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

.changeset/api-docs-page-header.md

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+---
+'@backstage/plugin-api-docs': patch
+---
+
+Hide the page header when rendered in the new frontend system, as the header is now provided by the `PageLayout` component.

.changeset/app-visualizer-subpages.md

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+---
+'@backstage/plugin-app-visualizer': minor
+---
+
+Migrated to use `SubPageBlueprint` for tabbed navigation and added a copy-tree-as-JSON header action using `HeaderActionBlueprint`. The plugin now specifies a `title` and `icon`.

.changeset/icon-element-migration.md

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+---
+'@backstage/frontend-plugin-api': minor
+'@backstage/frontend-app-api': patch
+'@backstage/core-compat-api': patch
+'@backstage/plugin-app-react': patch
+---
+
+Added `IconElement` type as a replacement for the deprecated `IconComponent`. The `IconsApi` now has a new `icon()` method that returns `IconElement`, while the existing `getIcon()` method is deprecated. The `IconBundleBlueprint` now accepts both `IconComponent` and `IconElement` values.

.changeset/page-layout-and-header-actions.md

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+---
+'@backstage/frontend-plugin-api': minor
+'@backstage/plugin-app': minor
+---
+
+Added `SubPageBlueprint` for creating sub-page tabs, `HeaderActionBlueprint` and `HeaderActionsApi` for plugin-scoped header actions, and `PageLayout` as a swappable component. The `PageBlueprint` now supports sub-pages with tabbed navigation, page title, icon, and header actions. Plugins can now specify a `title` and `icon` in `createFrontendPlugin`.

.changeset/ui-header-spacing.md

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+---
+'@backstage/ui': patch
+---
+
+Adjusted `Header` component spacing to remove bottom margins from the toolbar and tab wrapper.