Adds basic table of contents for mixins.less. You know for the kids.
Changes compiled assets after changing mixins.less
Adjusts some section titles to be more explicit.

FIXES #12992
Merge branch 'master' into mixin-table-of-contents

* master:
  Fix bad grunt merge and then G R U N T
  Reorder the Gruntfile a bit
  Fixes #12892: Link to one of the SO articles on Bootstrap modals and YouTube
  Remove unneeded quotes.
  Rename generateRawFilesJs to generateRawFiles
  Remove redundant parens

Conflicts:
	docs/assets/js/raw-files.min.js
	test-infra/npm-shrinkwrap.canonical.json
Reorganize mixins and G R U N T
Merge pull request #12993 from twbs/mixin-table-of-contents

Mixin table of contents