[ADDITION] Custom Subtitles

[anysad]

MERGE ONLY WHEN THE NEXT UPDATE IS OUT!

What does this PR add?

• This PR adds a page regarding custom subtitles that appear in the game.
• The page currently has two parts:
  • In-Game Subtitles, which appear during a song, like during Monster or Winter-Horrorland, and how to properly create and them.
  • Video subtitles, which appear in video cutscenes, where the page explains how to add subtitles to the video either via VLC Media Player or FFmpeg.