The npm package is named "popper.js" but the project is named "Popper", so use the latter consistently.

For some weird reason, using "Exports" as the callout header leads to TypeError coming from clipboard.js

Also add v4.5.0 in versions and keep README.md pointing to v4.5.0 so that there are no broken stuff.

- Broke up existing theming page into separate pages
- Audited and updated each new docs page (some pages include some TBD sections)
- Update sidenav to reflect changes
- Fix links that pointed to the old Theming page
- Update docs styles to reflect recent changes
- Rewrite some bits in the Migration page