Sticky container for docs layout

[bartaz]

Done

• add a sticky container to documentation layout to make table of contents sticky and scrollable
• drive-by: fix overflow in expandable side nav when collapsed

QA

• Open demo
• Review docs layout example
  • https://vanilla-framework-4895.demos.haus/docs/examples/layouts/docs
  • make sure table of contents on right is sticky when you scroll
  • adjust window height to make is smaller than the table of contents (or add some items to it in dev tools), make sure you can scroll the table of contents if it doesn't fit on screen

[webteam-app]

Demo starting at https://vanilla-framework-4895.demos.haus

[lorumic]

Looks good.