Fix integrating indexing and retrieval pipelines to FileIndex (#155)

* Add docs for settings
* Add mdx_truly_sane_lists to doc requirements

docs/pages/app/settings/user-settings.md

@@ -0,0 +1,52 @@
+# User settings
+
+`ktem` allows developers to extend the index and the reasoning pipeline. In
+many cases, these components can have settings that should be modified by
+users at run-time, (e.g. `topk`, `chunksize`...). These are the user settings.
+
+`ktem` allows developers to declare such user settings in their code. Once
+declared, `ktem` will render them in a Settings page.
+
+There are 2 places that `ktem` looks for declared user settings. You can
+refer to the respective pages.
+
+- In the index.
+- In the reasoning pipeline.
+
+## Syntax of a settings
+
+A collection of settings is a dictionary of type `dict[str, dict]`, where the
+key is a setting id, and the value is the description of the setting.
+
+```python
+settings = {
+    "topk": {
+        "name": "Top-k chunks",
+        "value": 10,
+        "component": "number",
+    },
+    "lang": {
+        "name": "Languages",
+        "value": "en",
+        "component": "dropdown",
+        "choices": [("en", "English"), ("cn", "Chinese")],
+    }
+}
+```
+
+Each setting description must have:
+
+- name: the human-understandable name of the settings.
+- value: the default value of the settings.
+- component: the UI component to render such setting on the UI. Available:
+
+  - "text": single-value
+  - "number": single-value
+  - "checkbox": single-value
+  - "dropdown": choices
+  - "radio": choices
+  - "checkboxgroup": choices
+
+- choices: the list of choices, if the component type allows.
+
+## Settings page structure