Sets the "render_fps" metadata used by the gym.wrappers.RecordVideo such
that, if it is used, the produced video will have the correct episode
length (i.e., the environment in the video will play at the correct
speed).

The correct render_fps should be equal to `1 / (self.cfg.decimation *
self.cfg.sim.dt)` which I integrated to the init function of the
ManagerBasedRLEnv. This seems to work well.

Fixes #892

## Type of change

- Bug fix (non-breaking change which fixes an issue)

## Screenshots

<img width="1048" alt="Screenshot 2024-08-28 at 18 10 30"
src="https://github.com/user-attachments/assets/d4fd8482-fbbc-48fa-8890-6a75a3fd99dc">

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there