docs: add user guide on Decibels app

docs/desktop/gnome/decibels.md

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+---
+title: Decibels
+author: Christine Belzie
+contributors: 
+---
+
+## Introduction
+
+Decibels is an app that plays audio files. Its user-friendly interface makes uploading and playing your favorite song(s) and recordings of lectures, project ideas, and other files easy.
+
+## Assumptions
+
+This guide assumes you have the following:
+
+- Rocky Linux
+- Flatpak
+- FlatHub
+
+## Installation Process
+
+Go to Flathub.org, type "Decibels" in the search bar, and click on **Install**.
+
+![Screenshot of the Decibels app page on FlatHub, showing the install button being highlighted by a red rectangle](/gnome/images/01_decibels_01.png)
+
+Then, run the following command in your terminal:
+
+`flatpak run.org.gnome.Decibels`
+
+## How to Use
+
+To use the app, do the following steps:
+
+1. Click on **Open**
+![Screenshot of Decibels' landing page with a red rectangle surrounding the blue open button](/gnome/images/02_decibels.png)
+1. Pick your desired file and click on the **Open** that appears in the upper right corner of the screen
+![Screenshot of Decibels file selection interface with numbered arrows indicating audio file and Open button](/gnome/images/03_decibels.png)
+1. Play away!
+![Screenshot of a QR code with instructions on how to share audio with another device](/gnome/images/audio-sharing_02.png)
+
+!!! note
+Tired of clicking with your mouse? Here are some ways you can use your keyboard to play and interact with your audio files
+
+- <kbd>CTRL</kbd> + <kbd>Shift</kbd>  + <kbd>O</kbd> = Open File
+- <kbd>Space</kbd> = Play/Pause
+- <kbd>← </kbd> = Move audio back 10 seconds
+- <kbd>→</kbd> = Move audio forward 10 seconds
+
+## Conclusion
+
+Eager to learn more about or have more ideas for this app? [Submit an issue in Decibel's repository at GitLab](https://gitlab.gnome.org/GNOME/Incubator/decibels/-/issues).