docs(docs): Add API Versioning ADR

[stevenj]

Description

This adds an ADR to describe our API versioning strategy and rules.

Description of Changes

Documentation only to add the new ADR.

Rendered Version: https://input-output-hk.github.io/catalyst-voices/branch/adr_api_endpoints/architecture/09_architecture_decisions/0007-api-versions/

Please confirm the following checks

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream module

[github-actions]

✅ Test Report | ${\color{lightgreen}Pass: 297/297}$ | ${\color{red}Fail: 0/297}$ |

[minikin]

That's a beneficial ADR, and it looks good to me overall.

[minikin]

LGTM