Update docs re. ECS Fargate image override

Hannes Hörlrequested to merge
hhoerl/521832-fargate-docs into main

What does this MR do?

Update the fargate driver docs:

  • Remove a dead link
  • Instead, add more context inline and link to to public blog with further info

Why was this MR needed?

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What's the best way to test this MR?

-

What are the relevant issue numbers?

closes: gitlab#521832 (closed)

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