docs: add comprehensive CONTRIBUTING.md guide

[AdekunleBamz]

Summary

Adds a complete contribution guide for the Stacks documentation repository.

Problem

The repository lacked a CONTRIBUTING.md file to guide new contributors on how to participate in improving the Stacks blockchain documentation.

Solution

Created a comprehensive contribution guide covering:

• Multiple ways to contribute to documentation
• Step-by-step contribution workflow
• Documentation standards and writing guidelines
• Review process and community engagement
• Support resources for contributors

Testing

• Verified all links and references are correct
• Ensured formatting follows markdown best practices
• Checked that the guide aligns with existing repository structure

Impact

This will help grow the contributor community by making it easier for developers, writers, and educators to contribute to Stacks documentation.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

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[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

SmartFlow Developer seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}