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

[Myriad-Dreamin]

Description

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

• The flat term should be translated to "展平", according to the MDN page.
• find all flat in the file, and annotate the them by specific syntax mentioned in the copilot instructions. If there is mistranslation, please fix them.
• for varaints like flatten, annotate them by 「被展平的」(#x-term-flat).
• 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 flat term in the docs/terms.toml file.