Add example to build_artifact docs

What does this MR do?

When I was reading the 13.6 release post about auto Ec2 deploy I was confused when looking at the docs as to what to write in the script for the build_artifact.

I watched the example video by @ebaque and saw that it should contain a script to build the artefact to deploy. In this case he used bundle exec jekyll build -d public.

I suggest adding this as an example in the docs to make it clear what is expected here.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team