Added the command that generates the API documentation schema file to…

[varshadr]

Fixes #2935

Technical details
Added the following command in the API README.md FILE:

./manage.py spectacular --color --file schema.yml

On running this command in mathesar_service_dev , will generate the API schema file (schema.yml) necessary for API documentation.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the develop branch of the repository
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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[silentninja]

Thanks for the PR. The command can be used in all the Mathesar environments and not just the development container, so I removed specifically mentioning mathesar_dev_service