MDEV-38844: 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.

[CLAassistant]

All committers have signed the CLA.

[gkodinov]

Thank you for your contribution. This is a preliminary review.
Please re-submit your change against 10.6 : we need to be updating this file across all active versions.
Also, please sort out the CLA bot requirements and make sure the CLA is cleared.

[luckyxhq]

Closing this PR as I've re-submitted the changes against the 10.6 branch as requested by the maintainers in #4661. The same formatting improvements are now targeting 10.6 for backporting across all active versions.