Make a top-level `Offset` article and group existing offset articles under it

[Walavouchey]

Split off from https://github.com/ppy/osu-wiki/issues/6080

Discussion on Discord has come to the conclusion that the offset articles would on their own reside in separate categories, but me and TicClick have agreed to instead group them together as subarticles under a more general offset article.

This article would provide an overview of what offsets are, what they do and what types of offsets exist in osu!, essentially replacing Disambiguation/Offset. Note that it would encompass all types of offsets (the timing kind, not spatial), with the basic concept of keeping things in sync in gameplay:

• Offsets that shift everything relative to the audio track (these three do the exact same thing)
  • Local offset
  • Universal offset
  • Online offset
• Beatmap offset (the "time between the start of the audio file and the song's first downbeat", i.e. the first red line) (not to be confused with beatmap offset in lazer, which is local offset renamed. i've also heard this being called "editor offset")
• Lead-in (the AudioLeadIn setting in a .osu file, which delays the start of the whole beatmap)
  • Perhaps a mention about storyboard intros, which effectively do the same thing
• Video offset (set in the .osu file, the delay from the audio track's start time to before a video starts)
  • This doesn't have an article, but it's mentioned in Disambiguation/Offset (and it doesn't really need one, but can be mentioned)

Feel free to express opinions on the scope of the article (it could be an even more general article on offsets in rhythm games, though that would require more effort and research)

General clean-up of the offset articles is also part of this (adding images and using more precise wording)

This is not supposed to be a guide or a technical explanation, just accurate and comprehensive descriptions of which offsets shift what things. I'll open up a pr of this sometime soon, because I think showing how all of this can be explained in a coherent way is best done by drafting an actual article.