Jekyll: exclude more files/folders.

We still can't exclude `./fonts/" and "./js/" due to https://github.com/jekyll/jekyll/issues/906
Merge pull request #11978 from XhmikosR/clean-up-_gh-pages

Jekyll: exclude more files/folders.
docs-assets: concatenate and minify all javascript files.

Also add the "min" suffix for the minified files.
Minify docs CSS too.