docs(cdk-stepper): add guide to create stepper

[Mokkapps]

• Added guide how to create a custom stepper using the CDK
• Added a material example to demonstrate the custom CDK stepper
• Made steps from CdkStepper public as it is necessary to build
  a custom stepper

[Mokkapps]

@mmalerba: This is the PR as discussed in #8993

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
     status "ci/circleci: api_golden_checks" is failing

If you want your PR to be merged, it has to pass all the CI checks.

If you can't get the PR to a green state due to flakes or broken master, please try rebasing to master and/or restarting the CI job. If that fails and you believe that the issue is not due to your change, please contact the caretaker and ask for help.

[Mokkapps]

CI fails due to a change in the stepper public API. How can I accept the new generated API as golden file? Is this something I have to do locally?

[mmalerba]

You can run the following command to accept:

bazel run //tools/public_api_guard:cdk_stepper_api.accept

[jelbourn]

I think this might be a bit too specific to be under Guides and instead makes more sense in the cdk component overview. @mmalerba thoughts?

[mmalerba]

@jelbourn I was thinking of it as a similar thing to this guide: https://material.angular.io/guide/creating-a-custom-form-field-control

The overview pages, at least the way they are today, are more of just discussing each part of the API and what you can do with it, rather than taking you through a more complex example step by step.

[ngbot]

Hi @Mokkapps! This PR has merge conflicts due to recent upstream merges.
Please help to unblock it by resolving these conflicts. Thanks!

[ngbot]

Hi @Mokkapps! This PR has merge conflicts due to recent upstream merges.
Please help to unblock it by resolving these conflicts. Thanks!

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