docs: update Vitest recommendations (#10621)
### Description
Updates the Vitest documentation to address GitHub Issue #10548. This
PR:
* Replaces deprecated Vitest "workspaces" with "projects" terminology
and configuration.
* Introduces a new `@repo/vitest-config` package for shared Vitest
configuration, eliminating relative path traversals and promoting
monorepo best practices.
* Updates example code snippets and `turbo.json` to reflect the new
package-based approach for improved cacheability and maintainability.
### Testing Instructions
Review the updated `docs/site/content/docs/guides/tools/vitest.mdx`
file.
* Verify that "workspaces" references are correctly replaced with
"projects".
* Ensure the new shared configuration package (`@repo/vitest-config`) is
clearly explained and demonstrated.
* Check the code snippets for accuracy and adherence to the recommended
monorepo structure.
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Co-authored-by: Cursor Agent <cursoragent@cursor.com>
