docs: Create new pipeline API - add link to array of hashes

What does this MR do?

There's some confusion around how to define variables correctly using the API. Whilst this is documented at the array of hashes page (which contains examples), this MR adds a link to that page from the Create new pipeline API page, in the hope people will find it a bit easier.

Related issues

gitlab-foss#52926 (closed)

#368653 (closed)

.. likely several StackOverflow questions

Author's checklist

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