[process] Document UX WG processes

We should have a document that explains the processes we use in the UX WG.
This document should be a markdown file and live in the repository.

Some of it is written under https://github.com/knative/ux/blob/main/CONTRIBUTING.md but more details are needed.

Things that I can think off:
- Design proposal process (in what tool, how to ask feedback, where, how, how to comment, difference among proposal versions, ...)
- Design handoff process (design is done, where do we store it, how do we apply it, ...)
- Surveys (where to post survey proposals, what tool to use for the survey, how to find users/contributors/targetPersona, ...)
- Roadmap board (what column is what, ...)
- How can an engineer use a visual for a social media post, documentation page, etc? Do I need to ask UX folks to prepare it for me, which is not very efficient? Or, if I want to modify an existing visual, how can I do it? This is important as we now want to use social media more actively in the Steering Committee.
- Overview of the repository (which is kind of empty right now) about where to put designs, where to put surveys, etc.
- ...

Note: README.md doesn't have a link to CONTRIBUTING.MD