docs: update formatter information (Prettier -> oxfmt)

[vinayaksodar]

Description

The documentation still references Prettier as the project's formatter, but the codebase has transitioned to using oxfmt (and oxlint).

Locations

• docs/CONTRIBUTING.md: Mentions "we use Prettier to format our code."
• docs/CONTRIBUTING_ADVANCED.md: Mentions "Code formatting is enforced by Prettier".

Expected Behavior

The documentation should accurately reflect that oxfmt is used for formatting and oxlint for linting, as seen in the package.json scripts and lint-staged configuration.