docs: add user guide for decoder app in GNOME section

docs/desktop/gnome/decoder.md

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+---
+title: Decoder
+author: Christine Belzie
+contributors: 
+---
+
+## Introduction
+
+Need a QR code for your website, app, and social media profiles? Check out  Decoder! :) It is an application that lets you can create, save, an export QR codes.
+
+## Assumptions
+
+This guide assumes you have the following:
+
+- Rocky Linux
+- Flatpak
+- FlatHub
+
+## Installation Process
+
+1. Go to the Flathub website, type "Decoder" in the search bar, and click on **Install**.
+![Screenshot of the install button being highlighted by a red rectangle](images/01_decoder.png)
+
+1. Then, run the following command in your terminal
+`flatpak run com.belmoussaoui.Decoder`
+
+## How to Create a QR Code
+
+There are two types of QR codes you can create on Decoder. Choose the option that best suits your needs:
+
+- [Text](#text)
+- [Wifi](#wifi)
+
+### Text
+
+1. Click on the **Text** button
+![Screenshot of a red arrow pointing next to a grey button with the word, Scan written in black font](images/02_decoder-text.png)
+1. Add a link to your desired website and add a description
+![Screenshot of a red arrow pointing upwards the text box, containing the description "Check out this song" with a Youtube link next to it ](images/03_decoder-text.png)
+1. Click on **Create**
+![Screenshot of a red arrow pointing next to a grey button that has a QR code symbol and the word "Create" underneath it. ](images/04_decoder-text.png)
+1. Click on **Save**
+![Screenshot of a red arrow pointing next to a grey button that has the word "Save" written in black. ](images/05_decoder-text.png)
+1. Click on **Export**
+![Screenshot of a red arrow pointing next to a grey button that has the word "Export" written in black. ](images/06_decoder-text.png)
+
+### Wifi
+
+1. Click on the **Wifi** button
+![Screenshot of a red arrow pointing at a grey button that has the word "Wifi" written in black.](images/01_decoder-wifi.png)
+1. Add the name of your network, password, and pick its form of protection from the encryption algorithm dropdown menu
+![Screenshot the wifi's name, password, and encryption algorithm being displayed. ](images/02_decoder-wifi.png)
+1. Click on **Export**
+![Screenshot of a red arrow pointing at a grey button that has the word "Export" written in black.](images/03_decoder-wifi.png)
+1. Click on **Save**
+![Screenshot of a red arrow pointing at a grey button that has the word "Save" written in black.](images/04_decoder-wifi.png)
+
+!!! tip
+
+    If your wifi network is hidden, click on the **Hidden** button.
+
+### How to Scan a QR code
+
+In addition to creating and generating QR codes, you can use Decoder to scan QR codes you saved on you computer. Here's how:
+
+1. Click on **Scan**
+![Screenshot of a red circle on a grey button that has the word "Scan" written in black.](images/01_decoder-scan.png)
+1. Click on  **From a Screenshot**
+![Screenshot of a red rectangle on a blue button that has the word "From a Screenshot" written in white.](images/02_decoder-scan.png)
+1. Pick your desired effects and click on **Take a Screenshot**
+![Screenshot of a grey square surrounding the options menu with a red arrow pointing towards it and the number one written on top. Next, there is a red square surrounding the "Take a Screenshot" button with a red arrow pointing upwards with the number two written underneath it.](images/03_decoder-scan.png)
+1. Click on **Share**
+![Screenshot of a red arrow pointing at a blue button that has the word "Share" written in white.](images/04_decoder-scan.png)
+1. Scan the QR code with your mobile device
+
+!!! note
+
+    To scan a QR code straight from your computer, ensure that you grant the app access to your computer's camera.
+
+## Conclusion
+
+Whether it's to share a restaurant's Wi-Fi with friends, grow your business, or network with other professionals at a conference, Decoder can ease the process of creating and scanning QR codes. Eager to learn more about or have more ideas for this application? [Submit an issue to its repository at GitLab](https://gitlab.gnome.org/World/decoder/-/issues).