docs: separate contributing pull request instructions

[MikeMcC399]

Description

Move the instructions to fork the repo into a new "Pull Requests" section of the CONTRIBUTING document.

Add a "Prerequisites" section.

Update GitHub documentation links.

Motivation and Context

The CONTRIBUTING document embeds instructions for forking the repo in the section Image Creation Manually, although forking is necessary for any type of contribution, not just for new images.

Testing Details

N/A

Types of changes

• Documentation
• Version change (Update, remove or add more Node.js versions)
• Variant change (Update, remove or add more variants, or versions of variants)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Other (none of the above)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING.md document.
• All new and existing tests passed.

[MikeMcC399]

Preview

• https://github.com/MikeMcC399/docker-node/blob/contributing-pull-requests/CONTRIBUTING.md