docs: side nav should be sticky so you can always navigate, even when scrolling

[codymullins]

The docs page is being used to "dog food" the new sidebar component. This would

1. Add support to the docs page sidebar to always float along the page
2. Optionally, build this support into the sidebar. (this can be done later as a separate issue)

Notes

• Can this be done with only Tailwind / CSS?
• If JS is required, need to document the impact of including this and potentially making it "opt-in"