Fix .form-inline docs example

Make sample code match live example
Move <label> out of .input-group into .form-group instead
Change field from Email to Username to make it less redundant, slightly more realistic
Add <label> for Username field for consistency
Fixes #15118
Merge pull request #15120 from twbs/fix-forms-doc

Fix .form-inline docs example
Use a 40x40 image for the `navbar` brand image example.

Saves 3.4KB.
Specify `img` dimensions in the HTML.
Tweak colors to improve accessibility.
Specify `img` alternative text.
Accessibility improvements.