Add documentation for all the API endpoints

[krashish8]

The documentation is already added, but it might not be correct and complete (because the main focus was on development).

Using swag for generating API documentation using annotaions.

• Check and modify the swagger-generated docs for all the API endpoints.
• Using go-openapi/runtime, the documentation can be served using Swagger UI, Redoc or Rapidoc (1, 2). Currently using Swagger UI (as default) on http://localhost:9100 and Redoc on http://localhost:9100/redoc

[dkastl]

Adding example could be also helpful: https://swagger.io/docs/specification/adding-examples/

[krashish8]

The documentation of all the API endpoints is almost done. Maybe we can add some detailed description about this application in the documentation, if that is possible in swagger.

Also added a sample (random) example for each endpoint, i.e. a prefilled value for each key-value pair, through which one can understand the type and format of the value required, or maybe directly make the request with the prefilled data.