Add tabs for code snippets and refactor API navigation

[andreslucena]

This PR adds the necessary code (JS and CSS) for having tabs:

And also adds the sidebar navigation for the proposal for the API docs:

This is to be merged with decidim/decidim#15804

[alecslupu]

👍

[andreslucena]

I think the build is just the Netlify deploy preview pipeline not working with the current configuration :/

I'll just merge this and see if the next build in develop actually deploys to production. On this case is "safe" to do the cowboy, as if it isn't deployed then Netlify rollbacks to the last working version (basically, the last commit in develop that had a successful build)