Add required features flags to the examples page

[Sigma-dev]

How can Bevy's documentation be improved?

Bevy examples sometimes require feature flags without mentionning them (ex: fps-overlay with the bevy_dev_tools flag).

This would make for a confusing situation when a developper copies the example code but the import can't get solved.

Solutions

My solution would be to display require feature flags somewhere on the page, be it in the header text and/or comments and/or a special place on the page that clearly marks what feature flags are necessary.

This data could either be added manually, or maybe a better way would be to automatically generate it based on the cargo.toml