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, ...