test: rework custom test command and example

[MikeMcC399]

• contributes to resolution of issue Add documentation for migrating from CLI #1440

Situation

The action parameter README > command allows executing a CLI command like npm run e2e:ci.

This example invokes a script defined in the project's package.json and runs it through a CLI call, bypassing the Cypress Module API call.

Users reading the documentation section command may come to the conclusion that the command parameter is a good way to use the action, even though the caution note attempts to explain the drawbacks that other related action parameters are ignored.

Change

• Make it clear when the action parameter command should be used
• Align the example in command with the example workflow .github/workflows/example-custom-command.yml
• Link to the section Migrating from CLI command
• Explain how the action (now) issues a warning if parameters which conflict with the command parameter are used together. (see PR feat: validate other parameters against command parameter when used #1445)

[cypress-app-bot]

• Create a Draft Pull Request if your PR is not ready for review. Mark the PR as Ready for Review when you're ready for a Cypress team member to review the PR.

[jennifer-shehane]

Thanks for making this clearer in each relevant spot!