Add brief note about justified navs being unsupported to fix #14598
Document that the autofocus attribute doesn't work in modals.

Closes #14478.
Mention HTML5 validator and Bootlint as part of troubleshooting steps

Fixes https://github.com/twbs/bootlint/issues/67
linkify reference to Nav component in Scrollspy docs
Also mention the "no justified navbars" limitation in the Nav docs

Refs #14598
[skip sauce]
Mention Bootlint in Getting Started docs under new Tools section

[skip sauce]
Merge pull request #14676 from twbs/mention-14598-more

Also mention the "no justified navbars" limitation in the Nav docs
Merge pull request #14675 from twbs/bug-report-guidelines

Mention HTML5 validator and Bootlint as part of troubleshooting steps
Merge pull request #14674 from twbs/fix-14478

Document that the autofocus attribute doesn't work in modals.
Merge pull request #14677 from twbs/bootlint-docs

Mention Bootlint in Getting Started docs under new Tools section