Make default installation directory name shorter

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What does this merge request do and why?

This MR sets the default installation directory name for GDK to gdk and updates the documentation accordingly. This is done because long paths can lead to errors like invalid argument: your socket path is likely too long, please change Gitaly's runtime directory during socket creation. See #2164.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

The updated documentation should be proofread.

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.

Closes #2204 (closed)