[Documentation] CHEF test devices example doc listed under guides

[greg-fer]

Documentation issues

The PR #27900 introduced a new CHEF text device example with related documentation.
There are two issues with this PR's documentation:

1. The example documentation README.md is accompanied by a user guide NEW_CHEF_DEVICES.md in the example directory.
2. The README.md for the example is mentioned in docs/guides/index.md instead of examples/index.md

These issues are causing inconsistencies in the doc structure and can lead to more serious discrepancies in the future if more PRs use #27900 as example.

Please fix these issues as follows:

• For 1., place the user guide under docs/guides and link it correctly in docs/guides/index.md. Make sure that the guide is pointing to the example. You can use the CHIP Tool guide's Source files section as example, where we point to the CHIP Tool example.
• For 2., remove the reference to the README.md from the guides section and add the mention of the example to examples/index.md.
• For both 1 and 2, reworking of both documents might be needed, so that they are complete on their own and cross-link adequately.

Platform

other

Anything else?

No response

[greg-fer]

@andy31415 , could you please fix this?