Add blog post testing instructions
What
This MR adds a new detailed testing plan for testing blog post changes as an issue template.
Based on this issue: https://gitlab.com/gitlab-com/gl-infra/delivery/-/work_items/21092+
Opened in favour of Add Patch Release blog post test issue template... (gitlab-com/gl-infra/delivery!21 - closed)
Why
Testing the blog post can be a complex process which can require quite a bit of attention to detail to ensure that no oopsies are made along the way.
Potential discussion points to address
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Is it too detailed? Are any steps unclear/missing? - more to come