Add support for VueJS

Since #407 (closed) we support modern JS which also means we can start benefitting from modern JavaScript frameworks such as VueJS.

VueJS is needed in order for us to use gitlab-ui in the docs project.

Benefits

Modern JavaScript frameworks have many benefits that we can take advantage of:

• Reduce code duplication by taking advantage of reusable components.

• Makes the docs codebase more maintainable.

• Reactivity (real-time updates without refreshing).

• VueJS takes advantage of the Virtual DOM, which essentially means faster rendering.

• Aligns with the product codebase, frontend engineers are familiar with VueJS.

• We can use gitlab-ui in the docs project.

• etc.