docs: describe manual PR image creation

[MikeMcC399]

• closes docs: misleading CONTRIBUTING invitation to update versions #2405
• supersedes docs: Add note about the bot opening PRs #2339 which was written before the automated description was added

Description

Instructions are added to the CONTRIBUTING document to clarify when it may be necessary to open a PR to create new images manually.

Motivation and Context

Originally the CONTRIBUTING document had no description of the automated process for image creation and it included a blanket invitation for users to submit update PRs.

The hierarchy of PR creation should be:

1. automated creation
2. manual creation by team member if automated creation fails
3. manual creation by other user if all else fails

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.