Fixes. Fixes 11918: Fork the @h5bp contributing guidelines and combine with our coding guidelines

BS3 commits: b8810fb75dea59615a700e47284103e029833692,
b59b37bc2cc52f3fba574cd03330d77de0be0ef2
mention core team maintenance & community contributors

BS3 commits: 07da1d731a7117e1281b43e6c79c441dc47e9e48
Docu: typo fix: replaced 'model-open' with 'modal-open'

BS3 commits: 85ba12932fc73218a21dade36af9221ec54b0fdf
Deleted curly brackets (in css.html)

BS3 commits: fb8310fbc6d413a3446ecf1772a77911d45fbd62
check that href id's are followed by valid char – fixes 10044

BS3 commits: 22f9767714a9f80d5d12479725e8875bdd83f341