Add Extend YunoHost section

Co-authored-by: ljf (zamentur) <zamentur@users.noreply.github.com>

pages/01.administer/12.extend/01.install_unpackaged_app/install_unpackaged_app.md

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+---
+title: Install unpackaged apps
+template: docs
+taxonomy:
+    category: docs
+routes:
+  default: '/install_unpackaged_apps'
+---
+
+!! This page is to be written

pages/01.administer/12.extend/02.configuration_management/configuration_management.md

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+---
+title: Install unpackaged apps
+template: docs
+taxonomy:
+    category: docs
+routes:
+  default: '/install_unpackaged_apps'
+---
+
+!! This page is to be written

pages/01.administer/12.extend/04.helpers/helpers.md

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+---
+title: App helpers
+template: docs
+redirect: '/packaging_apps_helpers'
+---

pages/01.administer/12.extend/05.hooks/hooks.md

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+---
+title: Hooks
+template: docs
+redirect: '/packaging_apps_hooks'
+---

pages/01.administer/12.extend/extend.md

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+---
+title: Understand and extend YunoHost
+template: docs
+taxonomy:
+    category: docs
+routes:
+  default: '/extend'
+---
+
+!! This section is to be written