Improve formatting and readability of new contributor guide

[henriquesantanati]

This commit enhances the formatting and readability of the new contributor guide. Changes include:

• Added a main header for the document
• Organized content into logical sections with subheaders
• Used bullet points for community joining instructions
• Numbered the steps for getting familiar with the contribution process
• Improved overall structure and visual hierarchy

These changes make the guide easier to read and follow for new contributors, potentially increasing engagement and participation in the community.

Which issue(s) this PR fixes:

Fixes #8152

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Hi @henriquesantanati. Thanks for your PR.

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