(Output Gateway) Remove apiary docs, dredd tests, improve readme

Co tenhle MR dělá? / What does this MR do?

Changed

• Improve README.md

Removed

• Apiary docs (permanently moved to OpenAPI) (general#498)
• Dredd tests

Související issues / Related issues

https://gitlab.com/operator-ict/golemio/code/general/-/issues/498+

Vývojářův kontrolní seznam / Developer checklist

• Ensure this change works
• Write unit tests for this change as needed
• Update the OpenAPI document if exists
• Ensure CI/CD pipeline passes
• Create or update the implementation documentation if necessary
• Include a changelog entry
• Request a review

Recenzentův kontrolní seznam / Reviewer checklist

• Ensure this change follows CODING-GUIDELINES and CONTRIBUTING
• Ensure this change follows the Golemio/OICT architecture
• Ensure this change does not add unnecessary dependencies (e.g. lodash)
• The changelog is updated
• The implementation documentation is created or updated if necessary
• The OpenAPI document is updated if exists