docs/frontend-system: rename plugin extension blueprints to common extension blueprints + fix links

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

docs/frontend-system/building-plugins/01-index.md

@@ -229,4 +229,4 @@ export const examplePlugin = createFrontendPlugin({
 
 The `ExampleEntityContent` itself is again a regular React component where you can implement any functionality you want. To access the entity that the content is being rendered for, you can use the `useEntity` hook from `@backstage/plugin-catalog-react`. You can see a full list of APIs provided by the catalog React library in [the API reference](../../reference/plugin-catalog-react.md).
 
-For a more complete list of the different kinds of extensions that you can create for your plugin, see the [extension blueprints](./03-extension-blueprints.md) section.
+For a more complete list of the different kinds of extensions that you can create for your plugin, see the [extension blueprints](./03-common-extension-blueprints.md) section.

docs/frontend-system/building-plugins/03-common-extension-blueprints.md

@@ -1,7 +1,7 @@
 ---
-id: extension-blueprints
-title: Frontend System Extension Blueprints
-sidebar_label: Extension Blueprints
+id: common-extension-blueprints
+title: Common Extension Blueprints
+sidebar_label: Common Extension Blueprints
 # prettier-ignore
 description: Extension blueprints provided by the frontend system and core features
 ---
@@ -22,7 +22,7 @@ An API extension is used to add or override [Utility API factories](../utility-a
 
 Components extensions are used to override the component associated with a component reference throughout the app. This uses an extension creator function rather than a blueprint, but will likely be migrated to a blueprint in the future.
 
-### NavItem - [Reference](../../reference/frontend-plugin-api.navitemblueprint)
+### NavItem - [Reference](../../reference/frontend-plugin-api.navitemblueprint.md)
 
 Navigation item extensions are used to provide menu items that link to different parts of the app. By default nav items are attached to the app nav extension, which by default is rendered as the left sidebar in the app.
 
@@ -38,7 +38,7 @@ Sign-in page extension have a single purpose - to implement a custom sign-in pag
 
 Theme extensions provide custom themes for the app. They are always attached to the app extension and you can have any number of themes extensions installed in an app at once, letting the user choose which theme to use.
 
-### Icons - [Reference](../../reference/frontend-plugin-api.ndleblueprint.md)
+### Icons - [Reference](../../reference/frontend-plugin-api.iconbundleblueprint.md)
 
 Icon bundle extensions provide the ability to replace or provide new icons to the app. You can use the above blueprint to make new extension instances which can be installed into the app.
 

docs/frontend-system/building-plugins/05-migrating.md

@@ -135,7 +135,7 @@ Then add the `fooPage` extension to the plugin:
 
 ## Migrating Components
 
-The equivalent utility to replace components created with `createComponentExtension` depends on the context within which the component is used, typically indicated by the naming pattern of the export. Many of these can be migrated to one of the [existing blueprints](03-extension-blueprints.md), but in rare cases it may be necessary to use [`createExtension`](../architecture/20-extensions.md#creating-an-extension) directly.
+The equivalent utility to replace components created with `createComponentExtension` depends on the context within which the component is used, typically indicated by the naming pattern of the export. Many of these can be migrated to one of the [existing blueprints](03-common-extension-blueprints.md), but in rare cases it may be necessary to use [`createExtension`](../architecture/20-extensions.md#creating-an-extension) directly.
 
 ## Migrating APIs