docs: add todo/placeholder pages under advanced

Placeholders for content that will be added in the future. Do not
publish these virtually empty pages.

docs/advanced/architecture.md

@@ -0,0 +1,21 @@
+---
+title: "Architecture"
+layout: default
+sort: 6
+published: false
+---
+
+# Architecture
+{: .no_toc }
+
+## Table of contents
+{: .no_toc .text-delta }
+
+1. TOC
+{:toc}
+
+---
+
+***WORK IN PROGRESS***
+
+This page first gives an architectural overview and describes principles behind.

docs/advanced/customization-guide.md

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+---
+title: "Customization Guide"
+layout: default
+sort: 5
+published: false
+---
+
+# Customization Guide
+{: .no_toc }
+
+## Table of Contents
+{: .no_toc .text-delta }
+
+1. TOC
+{:toc}
+
+---
+
+***WORK IN PROGRESS***
+
+This document explains with examples how to use hooks, feature files and the
+custom feature source.
+

docs/advanced/e2e-configuration-reference.md

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+---
+title: "E2E-Test Config Reference"
+layout: default
+sort: 7
+published: false
+---
+
+# End-to-End Test Configuration File Reference
+{: .no_toc }
+
+## Table of contents
+{: .no_toc .text-delta }
+
+1. TOC
+{:toc}
+
+---
+
+***WORK IN PROGRESS***
+
+This section describes the end-to-end test configuration file.

docs/advanced/worker-configuration-reference.md

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+---
+title: "Worker Config Reference"
+layout: default
+sort: 4
+published: false
+---
+
+# NFD-Worker Configuration File Reference
+{: .no_toc }
+
+## Table of contents
+{: .no_toc .text-delta }
+
+1. TOC
+{:toc}
+
+---
+
+***WORK IN PROGRESS***
+
+This section is a reference to all the configuration settings in the worker
+config file.