[ci skip] Update Gitaly installation documentation

What does this MR do?

Update Gitaly installation documentation. Now the rake task takes the path to the default storage directory.

What are the relevant issue numbers?

Closes

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides