Updating Transcode/Merging Videos

Hello @Garf, @alexandre-janniaux, this updates the Transcode/Merge_Videos as mentioned in #7 (https://wiki.videolan.org/VLC_HowTo/Merge_videos_together/)

I have divided it into three sections (case-III might feel a little redundant from a developer's point of view, but I added it to cover all bases from a user's perspective).

Also, I have mentioned that the "--sout-append-file" method works mostly for audio files using compatible codecs (as it did not work when I tested it for MP4, OGG (audio+video), and WEBM), but it worked for MP3, and OGG (audio only). Do review if the statement is true or should it be altered.