Recommend SSH for cloning gitlab-org/gitlab in GDK install docs

What does this merge request do and why?

Unauthenticated HTTPS clones of gitlab-org/gitlab are increasingly rejected by Gitaly under server load with a GitLab is currently unable to handle this request due to load error. This makes the default GDK installation path unreliable for GitLab team members.

This MR updates the install docs to:

1. One-line installation section: Add a note recommending SSH when prompted for a repo URL.
2. "Use GDK to install GitLab" section: Make SSH the explicitly recommended method with an explanation of why HTTPS is unreliable.
3. New troubleshooting section: Add "Clone fails with unable to handle this request due to load" under "Resolve installation errors"

How to set up and validate locally

1. Review the rendered documentation changes in this MR.
2. No functional code changes — documentation only.

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 MR references an issue describing the change.
• This change is backward compatible. If not, include steps to communicate to users.
• Tests added for new functionality. If not, raise an issue to follow-up.
• Observability added/updated (logging, metrics, tracing).
• Documentation added/updated.
• Announcement added for notable changes.
• gdk doctor test added.