Simplify code for 'cover' example using utility classes

[coliff]

This reduces the size of the CSS and (I think) makes the example source code easier to read.

Note; the background-color is very slightly different as I used bg-dark.

https://deploy-preview-28986--twbs-bootstrap.netlify.com/docs/4.3/examples/cover/

[MartijnCuppens]

We can remove this in favor of the .text-white utility:
https://github.com/twbs/bootstrap/blob/01af91124b738c96433ef22971abd06cad2ab2bd/site/content/docs/4.3/examples/cover/cover.css#L5-L10

We can use the .btn-light or maybe .btn-outline-light class here instead:
https://github.com/twbs/bootstrap/blob/01af91124b738c96433ef22971abd06cad2ab2bd/site/content/docs/4.3/examples/cover/cover.css#L12-L19

This css is overridden by .mt-auto:
https://github.com/twbs/bootstrap/blob/01af91124b738c96433ef22971abd06cad2ab2bd/site/content/docs/4.3/examples/cover/cover.css#L44-L46

We can use the responsive float classes here:
https://github.com/twbs/bootstrap/blob/01af91124b738c96433ef22971abd06cad2ab2bd/site/content/docs/4.3/examples/cover/cover.css#L70-L77

We can use .text-white-50 class here:
https://github.com/twbs/bootstrap/blob/01af91124b738c96433ef22971abd06cad2ab2bd/site/content/docs/4.3/examples/cover/cover.css#L92-L97

[XhmikosR]

Plus the .cover styles minus the button's vertical padding.

[coliff]

thanks for the good feedback @XhmikosR and @MartijnCuppens . I've further simplified the CSS and HTML.

[mdo]

Few more changes to simplify just a bit more, plus some nits :D.

[coliff]

Not really a big deal or anything, but can this one be backported to v4? I saw many of the other example code updates were backported so thought this might be worth doing too.

[MartijnCuppens]

We switched from jekyll to Hugo, so I don't know if we can just cherrypick this.

[XhmikosR]

Yeah, it didn't apply clean that's why I didn't cherry pick it.