Terraform Template: add custom cmd template job and recipe docs

What does this MR do and why?

This change sets adds a new .terraform:custom template job to the Terraform template. It's only very little and doesn't seem to add a lot of value, but I think it's worth it, because:

• we can add future configuration without the user having to change their job
• it gives confidence to the user that that's all the setup they need

In addition, this change adds a new doc page which documents "recipes" for how to use and configure the Terraform templates. It's meant that we and possibly the wider community adds further recipes which are worth documenting.

I've added the page to the navigation in gitlab-docs!3310 (merged)

MR acceptance checklist

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