Sidebar Mobile Support

[lamATnginx]

Proposed changes

Closes https://github.com/nginxinc/docs-platform/issues/466 and https://github.com/nginxinc/docs-platform/issues/490

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

[nginx-jack]

There's a few important!'s added in this PR, that should be removable by tweaking the specificity. For example, you could remove the important here with

nav.sidebar.sidebar__mobile-open {
  display: block;
}

.sidebar__mobile-open by itself, is only (0,1,1) specificity, but by adding the element and extra class, it increases to (0,2,1) which properly overrides the display: none triggered by

@media (max-width: 88rem) {
    .main-layout {
        & .sidebar-layout {
            nav {...`

It's long, but well worth the read - https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_cascade/Specificity