docs(GlSkeletonLoader): Add/improve story

What does this MR do?

This adds a story for the default usage of GlSkeletonLoader, and renames the existing story.

The existing story included extraneous DOM, presumably to frame the loader nicely. That's been removed, as it makes it less obvious what is part of the component and what isn't.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• [-] If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• [-] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• [-] If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • CustomersDot: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.