ci: :construction_worker: Build OpenAPI Specs & Postman collection as part of CI release job

[fergcb]

What does this do?

This PR deletes and stops tracking build artifacts such as OpenAPI specs and the Postman collection. Instead, these files are now generated during the deploy job of the CI pipeline, so that they can be included in releases - the Semantic Release config has been updated to do this.

Additionally, a step to validate the API spec has been added to the lint job, to ensure PRs etc. don't break the spec.

As part of these changes, build artifacts are now outputted to the dist/ folder, instead of being nested within the src/ folder, as they are not source code.

How was it tested?

The updated CI pipeline & scripts were used in my personal fork to generate this release.

Is there a Github issue this is resolving?

Resolves #673

Was any impacted documentation updated to reflect this change?

README updated to better explain how to obtain the Open API specs, etc.

Here's a fun image for your troubles