docs: Add detailed webserver documentation (#446)

## More detailed documentation

* **What is the goal of this PR?** 
Add more information about the exposed webserver.
* **What changes are included?**
Detailed documentation for the webserver endpoints
(`./docs/webserver-endpoints.md`)
Adding a table of content so it is easier to navigate directly to the
section you're interested on (Almost all `.md` files or at least all
those relevant)

## Additional Context

Not sure if this would get accepted but I thought it might be useful for
those trying to create separate apps that would sync files to the
device. It was at least to me trying to upload files using python as
stated
[here](https://github.com/crosspoint-reader/crosspoint-reader/discussions/434#discussioncomment-15545349)

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**PARTIALLY**_

USER_GUIDE.md

@@ -2,6 +2,27 @@
 
 Welcome to the **CrossPoint** firmware. This guide outlines the hardware controls, navigation, and reading features of the device.
 
+- [CrossPoint User Guide](#crosspoint-user-guide)
+  - [1. Hardware Overview](#1-hardware-overview)
+    - [Button Layout](#button-layout)
+  - [2. Power \& Startup](#2-power--startup)
+    - [Power On / Off](#power-on--off)
+    - [First Launch](#first-launch)
+  - [3. Screens](#3-screens)
+    - [3.1 Home Screen](#31-home-screen)
+    - [3.2 Book Selection](#32-book-selection)
+    - [3.3 Reading Mode](#33-reading-mode)
+    - [3.4 File Upload Screen](#34-file-upload-screen)
+    - [3.5 Settings](#35-settings)
+    - [3.6 Sleep Screen](#36-sleep-screen)
+  - [4. Reading Mode](#4-reading-mode)
+    - [Page Turning](#page-turning)
+    - [Chapter Navigation](#chapter-navigation)
+    - [System Navigation](#system-navigation)
+  - [5. Chapter Selection Screen](#5-chapter-selection-screen)
+  - [6. Current Limitations \& Roadmap](#6-current-limitations--roadmap)
+
+
 ## 1. Hardware Overview
 
 The device utilises the standard buttons on the Xtink X4 (in the same layout as the manufacturer firmware, by default):