Move cluster API docs to discussed place

[pipo02mix]

What this PR does / why we need it

Marian and I discussed where to located Cluster API content and we agreed on moving the introduction to the overview section meanwhile placing migration doc in the tutorials section

Towards https://github.com/giantswarm/giantswarm/issues/29613

Things to check/remember before submitting

• If you made content changes

  • Run make dev to render and proofread content changes locally.
  • Bump last_review_date in the front matter header if you reviewed the entire page.

[pipo02mix]

CI fails because the newer hugo version fails to render partials, fixing it in other PR