fix #10767; current .img-thumbnail is equivalent to old .img-polaroid
deprecate .hide; fixes #10446
move /assets/ to /docs-assets/ ; fixes #10715
avoid using the term "assets" due to potential confusion
Merge pull request #10769 from twbs/deprecate-hide

deprecate .hide; fixes #10446
Merge pull request #10770 from twbs/rename-assets

move /assets/ to /docs-assets/ ; fixes #10715
Merge pull request #10768 from twbs/polaroid-thumbnail-docs

fix #10767; current .img-thumbnail is equivalent to old .img-polaroid