Add brief note about justified navs being unsupported to fix 14598

BS3 commits: 5d051736a41b905ceca47b8121602db22c252576
linkify reference to Nav component in Scrollspy docs

BS3 commits: 93f4426fed83e53e60bc36a280687085eca1d4aa
Also mention the "no justified navbars" limitation in the Nav docs

BS3 commits: bfdd6d7683f12df15691fd1c81ed65315507ec53
Mention HTML5 validator and Bootlint as part of troubleshooting steps

BS3 commits: e4f8684d160f760993563342d9eaae4490978090
Document that the autofocus attribute doesn't work in modals.

BS3 commits: 6eb633775b92dedaf6381ef79e9760e45cfaf008
Mention Bootlint in Getting Started docs under new Tools section

BS3 commits: 73117814ea1d3312c242cea51f9d0cbf8764b550