Document open port requirement for runner to work in Gitpod

What does this merge request do and why?

• Add documentation regarding the requirement to open a port for runners when using Gitpod. Currently, the documentation only mentions opening a port when using the shell executor, but this is also required when using a docker setup.
• Add documentation regarding opening a port in the Gitpod VSCode UI.

I opened this because I was struggling with getting a runner to work when following the current documentation. The runner didn't work at first with fatal: Authentication failed for 'gitpod-url'. I also couldn't find the Remote Explorer tab mentioned in the documentation, so I used Gitpod documentation to figure out how to open the port.

How to set up and validate locally

1. Set up GDK on Gitpod following the guide.
2. Try setting up a runner using the docker option.
3. Try running a job. It will fail.
4. Once the items mentioned in this documentation are followed, the job passes.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.