Add overview dev portal page

[pipo02mix]

What this PR does / why we need it

Towards https://github.com/giantswarm/giantswarm/issues/30563

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]

@marians can you take a look plz?

[pipo02mix]

closing in favour of https://github.com/giantswarm/docs/pull/2238