Basic cleaning up of instructions and documentation about contributing to the grafana_dashboards as Opensource Product.
It should have:
- main as default and up to date branch 
- clear contribution guideline according to this template.
- PR template if needed and helpful
- in doc guideline and/or PR template pmm-review-fe should be mentioned as default assignee for the review.
See mongodb_exporter as an example.
- rename default branch to the main in github interface, so open PR would be also updated with it. For ppl's local branches, they need to update it, see mongodb_exporters old PRs for the example on how to.
- templates: https://drive.google.com/drive/u/1/folders/11LDjxM_s_xghcvgKNLLB9rPTrLVP4prP