Describe the Weblate REST API with OpenAPI

### Describe the problem

There is not a problem to solve per se. The [current API documentation is good](https://docs.weblate.org/en/latest/api.html). In my opinion, the proposed OpenAPI standard might be an improvement upon such documentation.

### Describe the solution you would like

[OpenAPI](https://swagger.io/specification/) (formerly known as Swagger) is one of the most popular specifications to describe REST APIs formally. It allows developers to define the entire Weblate REST API with a root OpenAPI document.

Some advantages this standard might offer over the current API documentation are:

- Make it easier for developers to code their own third-party REST clients to connect to the Weblate REST API, by using tooling which generates code automatically based on the OpenAPI document.
- Make it easier to document the Weblate REST API:
- The OpenAPI specification provides a standard way to document REST APIs. It should be easier to document the API when you can use a well-known syntax and keywords (structure), without having the API documentation written in free-form (`.rst` files).
- Sphinx seems to have plugins to integrate with OpenApi. If they support version 3.1 of the specification, it should be possible to display the OpenAPI document directly within the Weblate documentation.
- It should facilitate automating testing of the Weblate REST API.

### Describe alternatives you have considered

_No response_

### Screenshots

This is how the Weblate REST API looks in Swagger UI:

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In Sphinx with `sphinxcontrib-openapi`:

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In Sphinx with `sphinxcontrib-redoc`:

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### Additional context

Compared to Swagger UI, the Sphinx plugins are missing information and seem to work worse. If we can get `drf_spectacular` to work with Weblate, it should be possible to offer the OpenAPI document programmatically and letting people use it as they please.