[DOCS]: Create a Contributing Guidelines

[adiati98]

Description

There is no contributing guidelines in this repo. There are some instructions in the README. However, some of them need update.

As the instructions are long — and will get longer after update — it'd be clearer to move them to the contributing guidelines.

Relevant Documentation URL

https://github.com/mautic/developer-documentation-new/blob/5.x/README.md

Are you willing to help us fix this by making a pull request?

Yes

Suggestions (Optional)

• Create a contributing guidelines that lives in the .github folder.
• Move instructions from README to the guidelines
• Update the guidelines

Screenshots or Screen Recordings (Optional)

No response