feat(3128): Add docs for pipeline template job customization [3]

[tkyi]

Context

Users occasionally need to add more jobs to the pipeline, beyond those included in the pipeline template. Currently, this feature is not supported in the pipeline template.

Objective

This PR adds docs for user customization of existing pipeline template jobs as well as new user jobs in pipeline templates. Also fixes job template mergeSharedSteps example.

References

Related to https://github.com/screwdriver-cd/screwdriver/issues/3128

License

I confirm that this contribution is made under the terms of the license found in the root directory of this repository's source tree and that I have the authority necessary to make this contribution on behalf of its copyright owner.