feat(docs): document note source (viewing & editing)

docs/User Guide/User Guide/Advanced Usage/Note source.md

@@ -0,0 +1,55 @@
+# Note source
+## Understanding the source code of the different notes
+
+Internally, the structure of the content of each note is different based on the [Note Types](../Note%20Types).
+
+For example:
+
+*   [Text Notes](#root/_hidden/_options/_optionsTextNotes) are represented internally as HTML, using the [CKEditor](Technologies%20used/CKEditor.md) representation. Note that due to the custom plugins, some HTML elements are specific to Trilium only, for example the admonitions.
+*   [Code Notes](#root/_hidden/_options/_optionsCodeNotes) are plain text and are represented internally as-is.
+*   [Geo map](../Note%20Types/Geo%20map.md) notes contain only minimal information (viewport, zoom) as a JSON.
+*   [Canvas](../Note%20Types/Canvas.md) notes are represented as JSON, with Trilium's own information alongside with [Excalidraw](Technologies%20used/Excalidraw.md)'s internal JSON representation format.
+*   [Mind Map](../Note%20Types/Mind%20Map.md) notes are represented as JSON, with the internal format of [MindElixir](Technologies%20used/MindElixir.md).
+
+Note that some information is also stored as [Attachments](../Attachments). For example [Canvas](../Note%20Types/Canvas.md) notes use the attachments feature to store the custom libraries, and alongside with [Mind Map](../Note%20Types/Mind%20Map.md) and other similar note types it stores an SVG representation of the content for use in other features such as including in other notes, shared notes, etc.
+
+Here's part of the HTML representation of this note, as it's stored in the database (but prettified).
+
+```html
+<h2>
+	Understanding the source code of the different notes
+</h2>
+<p>
+	Internally, the structure of the content of each note is different based on the&nbsp;
+	<a class="reference-link" href="../Note%20Types">
+		Note Types
+	</a>
+	.
+</p>
+```
+
+## Viewing the source code
+
+It is possible to view the source code of a note by pressing the contextual menu in [Note buttons](../Basic%20Concepts/UI%20Elements/Note%20buttons.md) and selecting _Note source_.
+
+![](Note%20source_image.png)
+
+The source code will be displayed in a new tab.
+
+For some note types, such as text notes, the source code is also formatted in order to be more easily readable.
+
+## Modifying the source code
+
+It is possible to modify the source code of a note directly, however not via the _Note source_ functionality. 
+
+To do so:
+
+1.  Change the note type from the real note type (e.g. Canvas, Geo Type) to Code (plain text) or the corresponding format such as JSON or HTML.
+2.  Confirm the warning about changing the note type.
+3.  The source code will appear, make the necessary modifications.
+4.  Change the note type back to the real note type.
+
+> [!WARNING]
+> Depending on the changes made, there is a risk that the note will not render properly. It's best to save a revision before making any big changes.
+> 
+> If the note does not render properly, modify the source code again or revert to a prior revision. Since the error handling for unexpected changes might not always be perfect, it be required to refresh the application.