Deduplicate documentation

[RichDom2185]

We have multiple sources of truth for some documentation. These are mainly duplicated by js-slang documentation (due to the stubs). For example,

• Runes:
  1. https://docs.sourceacademy.org/RUNES
  2. https://source-academy.github.io/modules/documentation/modules/rune.html

Actually, there's a lot of duplication for stuff that are now inside modules (e.g. runes, sounds). I suspect these are remnants from when they were still implemented as external libraries:

See also: https://github.com/source-academy/js-slang/blob/master/scripts/jsdoc.sh