Update Frontend documentation regarding Vue and Icons/Illustrations

What does this MR do?

• Clean up the Icons/Illustrations page
  • Navigation was broken due to two top-level headings
  • Moved the contents of "Component Gotchas" from the Vue page to this page
  • Clean up examples with multiline code blocks and syntax highlighting
  • Made function parameter documentation a bit more readable by using lists
• Vue page:
  • Typos
  • Cleaned up document structure, by using correct heading levels
  • Cleaned up example code for application and tests
  • Used axios-mock-adapter in the test example
  • Use Vue.nextTick where needed and removed superfluous setTimeouts
  • Added vm.$destroy() to the afterEach example

Comparison

Page	Current	Future
Icons	https://docs.gitlab.com/ce/development/fe_guide/icons.html	http://ce-docs-vue-guide-update.178.62.207.141.xip.io/ce/development/fe_guide/icons.html
Vue	https://docs.gitlab.com/ce/development/fe_guide/vue.html	http://ce-docs-vue-guide-update.178.62.207.141.xip.io/ce/development/fe_guide/vue.html

Does this MR meet the acceptance criteria?

• Conform by the code review guidelines
  • Has been reviewed by a Frontend maintainer