Review `locales/docs/typst-docs.toml` - fold term

[Myriad-Dreamin]

Description

Continue reviewing the locales/docs/typst-docs.toml.

• The fold term should be translated to "折叠". 在函数式编程中，折叠（fold），也称为归约（reduce）、积累（accumulate）、聚集（aggregate）、压缩（compress）或注入（inject） https://zh.wikipedia.org/wiki/Fold_(%E9%AB%98%E9%98%B6%E5%87%BD%E6%95%B0)
• find all fold in the file, and annotate the them by specific syntax mentioned in the copilot instructions. If there is mistranslation, please fix them.
• again, please following the copilot instructions, including but not limited to:
  • only the first term in a translation text is annotated.
  • also add or update the fold term in the docs/terms.toml file.