refactor!: remove EnhancedSpy type, deprecate SpyInstance, improve mocks and vi documentation

[sheremet-va]

Description

This PR

• removes EnhancedSpy because it is an implementation detail and is no longer needed
• deprecates SpyInstance in favor of MockInstance
• adds inline documentation to "vi" object
• adds inline documentation to mock instance
• adds more documentation in general
• updates vitepress to the latest version which changes design a little bit

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Tests

• Run the tests with pnpm test:ci.

Documentation

• If you introduce new functionality, document it. You can run documentation with pnpm run docs command.

Changesets

• Changes in changelog are generated from PR name. Please, make sure that it explains your changes in an understandable manner. Please, prefix changeset messages with feat:, fix:, perf:, docs:, or chore:.

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