Update/doc concepts

[KaiWaldrant]

Describe your changes

mention api file formats specification in concepts page

Checklist before requesting a review

• [x] I have performed a self-review of my code

• Check the correct box. Does this PR contain:

  • [ ] Breaking changes
  • [ ] New functionality
  • [ ] Major changes
  • [x] Minor changes
  • [ ] Bug fixes

• [ ] Proposed changes are described in the CHANGELOG.md

• [ ] CI Preview succeeds and looks good!

[github-actions[bot]]

[KaiWaldrant]

Already merged #280