Update custom plugin guide CI references

[ekamran]

Summary

Updates the custom plugin guide so its generated file list matches the current wp scaffold plugin documentation.

Changes

• Replace the stale .travis.yml reference with .circleci/config.yml.
• Update the CI description from Travis CI to CircleCI.
• Keep the existing --ci=<provider> guidance for alternate CI providers.

Verification

• Confirmed wp scaffold plugin docs list .circleci/config.yml as the default generated CI configuration.
• Confirmed wp-cli/scaffold-command documents CircleCI as the default CI provider.
• Ran git diff --check.
• Ran composer lint.
• Ran composer phpcs.
• Ran composer phpunit.
• Ran composer behat (skipped: no behat.yml).

Note: composer phpstan currently fails on existing type errors in bin/Handbook_Command.php unrelated to this documentation-only change.

[swissspidy]

We could make this more future proof by just listing some of the generated files as an example, then we don't have to update this page every time the command changes.

[ekamran]

We could make this more future proof by just listing some of the generated files as an example, then we don't have to update this page every time the command changes.

Good point. I updated the wording to avoid presenting the generated files as an exact full list.

Instead, the guide now lists a few example files with “files such as” and describes the test/CI output more generally. That should make the page less likely to go stale if wp scaffold plugin changes the exact generated files or default CI provider again.