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

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: henriquesantanati Once this PR has been reviewed and has the lgtm label, please assign nikhita for approval. For more information see the Kubernetes Code Review Process.

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Hi @henriquesantanati. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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