- Added a services constraint document mentioning that a service's methods must be async.
- Modify the code in the services intro document to use `async` method.
- Replace remote query documentation with new Query documentation
- Add redirect from old remote query to new query documentation
- Update remote query usages across docs to use new query usage.
- Add new documentation pages for API route intro, responses, errors, and validation
- General fixes and improvements across existing API route guides
> Note: I didn't work on the guide for additional data validation. Will work on this as part of my next focus on workflows.
- Added documentation on how to use Medusa's tools from the `medusa-test-utils` package to create integration and unit tests.
- Added a manual reference on the `medusaIntegrationTestRunner` and `moduleIntegrationTestRunner` functions. Since the typings in the source code aren't very informative, I opted for a manual reference shedding light on the important bits.
Closes DOCS-852
