jellyfin/ffmpeg: added a guide to concatenate videos

wiki/linux/ffmpeg.md

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+# ffmpeg
+
+[ffmpeg](https://www.ffmpeg.org) is a free and open-source suite consisting of
+many audio and video tools and libraries.
+
+## Concatenate multiple video files with matching audio-tracks
+
+First you need to create a text file `files.txt` that includes a line for every
+video you want to concatenate in the form of:
+
+```shell
+file '<path to the video>'
+```
+
+Make sure the videos are in correct order.
+To concatenate these video files you can run:
+
+```shell
+ffmpeg -f concat -safe 0 -i files.txt -map 0 -c copy output.mp4
+```