Update icons documentation

What does this MR do?

This updates the documentation located at https://docs.gitlab.com/ee/development/fe_guide/icons.html

The Icons > Usage in Vue section has been updated to recommend using GitLab UI's component instead of the one located in GitLab's shared Vue components.

Also contains a few minor formatting improvements: line breaks between </script> and <template>, use html syntax highlighting for Vue SFC code examples, ...

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content