Add pipeline watch architecture design

[luizbafilho]

Closes https://github.com/weaveworks/pipeline-controller/issues/191.

[squaremo]

@luizbafilho and I had a talk about having a look at the whole state as mentioned in https://github.com/weaveworks/weave-gitops-private/pull/128#discussion_r1300092044. Outcome:

• we'll figure out how to rework the proposal
• we need to come up with an scheme for calculating promotions

On that second one, the main problems are:

• how do you decide when there's a revision to promote (or to test against the gates, e.g., health check)
• some promotions involve poking a webhook or creating a PR -- how do you ensure these are only completed once

[squaremo]

Had an initial look with a few suggestions

Thanks for your thoughtful suggestions @enekofb. I've asked for guidance on a couple of points you made, but I'll be able to make progress without.