Modernize docs app and migrate to Bootstrap 5

[simonihmig]

Unfortunately I have done a poor job of (not) separating different issues here, so the diff got quite large. So this

• updates the docs app to BS5
• fixes related markup issues
• updates example snippets to more modern Octane-based code
• fixes deprecations (in e-b, but also things like missing this. in templates)
• migrates most controllers to native classes

@jelhan feel free to take a look! But this were mostly "mechanical" changes, so a super thorough review is probably not needed...