docs v5: switch to ES6

[XhmikosR]

Hugo v0.70.0 introduced the ability to use Babel, which means we can finally use ES6 everywhere in the codebase for v5.

There's still a bug that needs to be fixed, but should only affect Windows: gohugoio/hugo#7251

[hembhagat99]

Hello @XhmikosR I would like to start contributing to Bootstrap. I went through the Contributing Guidelines and found that for issues with label help wanted you would love to get help from community.

I would like to contribute in this issue.

To better understand issue, I would like to have more insights of the issue.

[Bam92]

I was about to open such an issue, but I found this to be very similar to mine.

I've read the source code for bootstrap.js and I've noticed that you're still using an old fashioned JS style. So, I wish I helped to start the migration to ES6+. I've seen that there's a PR, but for the doc only. I wish I helped w/ bootstrap.js. Any comment?