Refactor Contributing Guideline doc

[FilippoZazzeroni]

:scroll: Tickets

Jira ticket Github issue

:bulb: Description

Refactor Contributing Guideline doc to add more info on how to Contribute and to change the doc structure in order to prioritize the Looking for issue section.

:pencil: Checklist

You have to check all boxes before merging

• [x] Filled in the above information (tickets numbers and description of your work)
• [x] Updated the PR name to follow our PR naming guidelines
• [ ] Wrote unit tests and/or ensured the tests suite is passing
• [ ] When working on UI, I checked and implemented accessibility (minimum Dynamic Text and VoiceOver)
• [ ] If needed, I updated documentation / comments for complex code and public methods
• [ ] If needed, added a backport comment (example @Mergifyio backport release/v120)