fix: consolidate template README images into single directory

[Parkreiner]

Description

This PR moves all the existing template README images for the Coder namespace from being defined inline to its .images directory. This makes the image-processing logic in the Registry build step easier to maintain.

Type of Change

• New module
• Bug fix
• Feature/enhancement
• Documentation
• Other

Testing & Validation

• Tests pass (bun test)
• Code formatted (bun run fmt)
• Changes tested locally

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Related Issues

Related to #132

[matifali]

@Parkreiner can we also add linting to enforce this for all future templates/modules.

[Parkreiner]

@matifali I agree that we need it

The main problem is that the templates were brought into the repo without updating any of the validation logic to be aware that templates exist at all. I'd rather make a separate PR that addresses the template validation all in one go

[matifali]

@matifali I agree that we need it

The main problem is that the templates were brought in without updating any of the validation logic to be aware of templates at all. I'd rather make a separate PR that addresses the template validation all in one go

A separate PR is fine. Please create an issue for tracking. Thank you.

[Parkreiner]

@matifali Done #194

[Parkreiner]

Got rid of all these extra dividers because GitHub already automatically adds dividers for h2 headers