Version 4: Develop the branching strategy

### Summary
Determine and implement the branching strategy for when we start working on v4 alongside v3.

### 💬 Description
We need to set up separate develop and main branches for when we start working on v4.

Actions:
- Create a develop and main branch for v4 features - could probably just be called "v4.0.0/develop" and "v4.0.0/main" or similar
- Ensure any necessary workflows we have for v3 are duplicated for v4, e.g. the checks when we open PRs
- The release workflows can be left for now as they will be done as part of #4167

**Note: this will need to be done in both the ic-ui-kit and ic-design-system repos**

Compare the existing v3 branches and pull-request check workflows in both the ic-ui-kit and ic-design-system repositories. Define the v4 develop and main branch names, duplicate the necessary v3 checks for those branches, and leave release workflows for #4167. Done means both repositories have the agreed branches and working PR checks.

Written by the indexing model from the issue text.