remove current HTML-based symbol, add any missing aria-* attributes

/fyi @mdo

[skip sauce]

It's unclear why the example currently has the content that's
shown/hidden BEFORE the toggle. Generally, this is not a recommended
content order, as after toggling, the newly shown content precedes the
toggle and requires keyboard users, and particularly assistive
technology users, to then navigate in reverse to reach it.

[skip sauce]

[ci skip]

Because of a color scheme has a color and background color.

Change d6b6a20a missed two doc mentions
of the previous names when changing to new:
    .navbar-default  -->  .navbar-light
    .navbar-inverse  -->  .navbar-dark

Related to #17314.

rewrite navbar once more for lower key styles and a new take on modifiers for improved customization

- Simpler main nav on all pages
- Back to purple masthead on homepage instead of dark graphite
- Active link styles on the main nav
- Cleaned up sidebar nav
- New docs layout name
- Homepage copy edits
- Updated bright purple docs color