Update grafana_dashboards branching and guideline

The Goal:

Basic cleaning up of instructions and documentation about contributing to the grafana_dashboards as Opensource Product.

It should have:

1. main as default and up to date branch [1]
2. clear contribution guideline according to this template.
3. PR template if needed and helpful
4. 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. 

Hints:

1. 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.
2. templates: https://drive.google.com/drive/u/1/folders/11LDjxM_s_xghcvgKNLLB9rPTrLVP4prP