[Docs] Unify custom plugin documentation and examples, and modify code

[TakhyunKim]

TanStack Devtools version

v0.6.14

Framework/Library version

N/A

Describe the bug and the steps to reproduce it

• https://tanstack.com/devtools/latest/docs/framework/react/guides/custom-plugins
• https://tanstack.com/devtools/latest/docs/framework/react/examples/custom-devtools

The counter.ts example code in the custom plugins documentation differs from the counter.ts in the example code.
The location for incrementing or decrementing the count must be standardized.

Additionally, the example for the custom plugin increments or decrements count after history.push(count), so it seems we need to reverse this order.

Your Minimal, Reproducible Example - (Sandbox Highly Recommended)

N/A

Screenshots or Videos (Optional)

custom-plugins docs example code

custom devtools docs example code

Do you intend to try to help solve this bug with your own PR?

Yes, I am also opening a PR that solves the problem along side this issue

Terms & Code of Conduct

• I agree to follow this project's Code of Conduct
• I understand that if my bug cannot be reliable reproduced in a debuggable environment, it will probably not be fixed and this issue may even be closed.