CSS Containment

[estelle]

• CSS Containment Module page
  mdn/content#31950
  • New landing page
  • brief (5-paragraph approx) description
  • demonstration example
  • properties
  • data types
  • associated properties
  • glossary terms
  • https://drafts.csswg.org/css-contain/
• Convert current "module" to containment guide.
• Using container queries guide
• Update container style queries on @container page

OTHER:

• New Using container style queries
• Move block formatting context guide to be under display mdn/content#31997
• Add three container guides and BFC guide to Yari Sidebar