RuntimeSDK: reference generated OpenAPI specs in the documentation

[sbueringer]

The current RuntimeSDK documentation (which will be added by #6756) does still include links to
the OpenAPI specs (for Swagger UI) that we generated for our proposals.

We already include a runtime-sdk-openapi.yaml as part of our releases. Unfortunately, the links available for GitHub release
artifacts cannot be used directly with Swagger UI like this:

Works:
https://editor.swagger.io/?url=https://raw.githubusercontent.com/kubernetes-sigs/cluster-api/main/docs/proposals/images/topology-mutation-hook/runtime-sdk-openapi.yaml
Fails:
https://editor.swagger.io/?url=https://github.com/kubernetes-sigs/cluster-api/releases/download/v1.2.0-beta.1/runtime-sdk-openapi.yaml

Goal of this issue is to find a way to do it anyway :).

One (relatively) easy option would be to:

• generate the runtime-sdk-openapi.yaml via a make target (like e.g. our CRD YAMLs)
• generate it to a location which is part of the book
  • If this doesn't work we can probably also point to the raw file of a specific branch via raw.githubusercontent
• use the location to those YAMLs in the documentation
  • we should figure out a way that each version of the book uses the right link. This could also be hard-coded, just like we have to bump the "release link" snippets today to use the correct CAPI version
• enforce that the YAML is up-to-date by making it part of make verify-gen

/kind documentation
/area runtime-sdk