docs: Add ffmpeg installation guide

docs/help/ffmpeg.md

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+FFmpeg is required for thumbnail previews and playback features on audio and video files. FFmpeg is a free Open Source project dedicated to the handling of multimedia (video, audio, etc) files. For more information, see their official website at [ffmpeg.org](https://www.ffmpeg.org/).
+
+# Installation on Windows
+## Prebuilt Binaries
+Pre-built binaries from trusted sources are available on the [FFmpeg website](https://www.ffmpeg.org/download.html#build-windows).
+
+To install:
+1. Download 7z or zip file and extract it (right click > Extract All)
+2. Move extracted contents to a unique folder (i.e; `c:\ffmpeg` or `c:\Program Files\ffmpeg`)
+3. Add FFmpeg to your PATH 
+    1. Go to "Edit the system environment variables"
+    2. Under "User Variables", select "Path" then edit
+    3. Click new and add `<Your folder>\bin` (e.g; `c:\ffmpeg\bin` or `c:\Program Files\ffmpeg\bin`)
+    4. Click okay
+
+## Package Managers
+FFmpeg is also available from:
+
+1. WinGet (`winget install ffmpeg`)
+2. Scoop (`scoop install main/ffmpeg`)
+3. Chocolatey (`choco install ffmpeg-full`)
+
+# Installation on Mac
+## Homebrew
+FFmpeg is available via [Homebrew](https://brew.sh/) and can be installed via;
+
+`brew install ffmpeg`
+
+# Installation on Linux
+## Package Managers
+FFmpeg may be installed by default on some Linux distributions, but if not, it is available via your distro's package manager of choice;
+
+1. Debian/Ubuntu (`sudo apt install ffmpeg`)
+2. Fedora (`sudo dnf install ffmpeg-free`)
+3. Arch (`sudo pacman -S ffmpeg`)
+
+# Help
+For additional help, please join the [Discord](https://discord.gg/hRNnVKhF2G) or create an Issue on the [GitHub repository](https://github.com/TagStudioDev/TagStudio)