Update documentation links from referencing 3.0.2 to 3.1.0

[kdipippo]

This repo's spec is in OpenAPI 3.1.0: https://github.com/Redocly/openapi-starter/blob/d6c17acb15dab2f44a3e64a54b1c0c257180e93e/openapi/openapi.yaml#L1

But the links here are to the 3.0.2 markdown file when 3.1.0.md exists: https://github.com/Redocly/openapi-starter/blob/d6c17acb15dab2f44a3e64a54b1c0c257180e93e/openapi/openapi.yaml#L22

It may be enough to change the links and be done with it, but I am unsure if there's also other parts of this template that need a 3.0.2 -> 3.1.0 upgrade.

[kdipippo]

Created https://github.com/Redocly/openapi-starter/pull/118

EDIT: I accidentally closed this PR. So resolving this issue means replicating the work I performed in there.