Draft: docs: enhancement of the data plane signaling protocol doc

[f-zimmer]

What this PR changes/adds

The PR adds some further info about the Data Plane Signaling, especially in regards to the Data planes internal state machine and the message layout.

Why it does that

After having had a first read of the current state of the DPS doc, it was hard to understand how the messages are structured, where to find them and what the actual interaction between CP and DP would look like. After having crawled through the source code I enhanced the doc by adding my newly gained insights in order to keep others from needing to do the same.

Further notes

Added the diagrams as plantuml in order to be easily maintainable in the future.

Linked Issue(s)

None

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[github-actions]

We are always happy to welcome new contributors ❤️ To make things easier for everyone, please make sure to follow our contribution guidelines, check if you have already signed the ECA, and relate this pull request to an existing issue or discussion.

[paullatzelsperger]

I understand that this PR is still in draft, but I strongly recommend familiarizing yourself with our contribution guidelines, that explicitly state "No surprise PRs".

Unless you formulate your intent in a discussion or an issue, clearly outlining pain points, motivating the change and a drafting a proposed solution, the chances of anyone reviewing this are slim, to say the least.

[edit]: I can already tell you that "because I found it too hard to understand" is not a good-enough reason.

The fact that you are proposing changes to spec documents raises the bar even more.