Improve CONTRIBUTING.md formatting and consistency

[luckyxhq]

Description

This PR improves the CONTRIBUTING.md file by fixing formatting inconsistencies and updating an HTTP link to HTTPS.

Changes Made

• Standardized header underlines to use 3 dashes consistently (some used 4-5 dashes)
• Removed double spaces before links throughout the document
• Added missing space after period on line 22
• Updated http://mariadb.meetup.com/ to https:// for security

Rationale

• Consistency: Standardized formatting makes the document easier to read and maintain
• Professional appearance: Proper spacing and formatting reflect well on the project
• Security: HTTPS provides encryption and is the modern standard
• Markdown standards: Consistent header underlines are a markdown best practice

These changes improve document readability and follow markdown best practices.

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