docs: redirect root pages to the latest version

Like /about/ to 4.2/about/ etc.

Here's the list

about
browser-bugs
components
css
customize
examples
    blog
    carousel
    cover
    dashboard
    grid
    jumbotron
    jumbotron-narrow
    justified-nav
    navbar
    navbar-fixed-top
    navbar-static-top
    non-responsive
    offcanvas
    signin
    starter-template
    sticky-footer
    sticky-footer-navbar
    theme
    tooltip-viewport
getting-started
javascript
migration -> to /docs/3.4/migration/ ?