Draft: docs: adding mermaid sequence diagram displaying the current picture

I'm opening this MR to facilitate discussions around adding a sequence diagram. This will help us better understand the runtime flow and interactions between various actors involved in the project. The diagram won't aim to exhaustively define internal validation logic, but rather illustrate the relationships and calls required for git-eca to complete its task effectively.

requested review from @malowe and @gnugomez

@malowe I applied the notes you shared with me as best as I could. If you have time, please take a look.

That looks close enough for the most part, though I'd update GitLab Pre-receive Hook to be Other ECA validation requests. There are some GitHub actions that call the validation outside of the context of GitHub pull requests. It's weird, but it's a case that is used in enough places to bear mentioning.

I'd also update a few other things, namely the name of the h2 to Current general validation flow and to update the TOC at the top of the file. There's a neat plugin for VS Code called Markdown All in One that can take care of Tables of contents for you which is super handy.

Nice to hear that!

I'll apply what you mentioned, and I think I'll also be adding the Mermaid diagram VS Code extension to the list.