Provide link to GDK instructions

What does this MR do and why?

Without encouraging folks to use GDK first to do GitLab documentation contributions, if they already use GDK successfully now, it's worth them using GDK for GitLab Docs.

A companion MR to: gitlab-development-kit!3404 (merged).

CC @ashmckenzie @nao.hashizume @dianalogan

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