Introduce CI/CD dev guides

What does this MR do?

As first contributor myself, when joined the company, I found challenging to know where in the code base this feature is, or where the processing start. Luckily I had a few video sessions with team members that guided me through some of the most important parts of the CI/CD code.

I think our group lacks of domain specific development documentation which would help lowering the barrier when contributing for the first time (especially community contributors).

This is the first of a series of documentation pages. I tried to set up a quick structure of the page for the upcoming docs but I very much think that this will become an index/overview page for more specific doc pages.