Add required features flags to the examples page

## How can Bevy's documentation be improved?

Bevy examples sometimes require feature flags without mentionning them (ex: [fps-overlay](https://bevyengine.org/examples/dev-tools/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](https://github.com/bevyengine/bevy/blob/a35eed0ea4e5739a668e5661cddc87f69750484d/Cargo.toml#L3860)