[docs]: clarify CostService usage and improve metered services discoverability

[TanCodeX]

What does this PR do?

This PR improves and clarifies the documentation around Metered Services and CostService, which are key to Puter's open-source usage-based billing system.

Included in this PR:

• Improved structure and content in src/backend/doc/features/metered-services.md
• A reference link in puterai/README.md (intro section) to direct users to the metered services doc
• A short pointer in src/backend/README.md to guide developers toward backend documentation

These changes ensure contributors can easily find and understand how Puter handles usage billing through CostService.

Fixes #1282

Type of Change

• Bug fix
• Documentation update
• New feature
• Refactor
• Breaking change

How Should This Be Tested?

• Confirm the link in puterai/README.md is visible and points correctly
• Open src/backend/README.md and verify the reference to metered services doc is included
• Review the updated metered-services.md for clarity, accuracy, and structure

Checklist

• I’ve read the contributing guide
• My updates follow the style and documentation conventions of this project
• I’ve commented where needed for clarity
• No build errors or warnings introduced
• All necessary documentation updates are included

[KernelDeimos]

I just noticed another problem - not with this PR, but an aspect the way we do documentation in general.

You added a README.md file to the module. That makes sense - we should have that there. We should have that there because it's what people will expect if they're browsing the source via GitHub. However, the way we generate the wiki from markdown files in source (tools/genwiki) expects markdown files to be in doc/. I'm experiencing a bit of decision paralysis on exactly how to deal with this.

The README.md should also be more general, but since this PR focuses on documenting cost it makes sense to create the new file with a focus on that for the moment. This will need to be updated later.

I'll make issues tomorrow addressing these concerns. This PR is an overall improvement despite these concerns so I'm going to merge it.

[TanCodeX]

@KernelDeimos Thanks for merging! Feel free to assign me the new issue when it's created. Happy to help.