Adds RSL RL installation instructions for pip (#1770)

# Description

Due to limitations from PyPI, all dependency packages have to be
published pip packages.
Unfortunately, RSL RL currently does not have a pip package, so we could
not include it into the Isaac Lab package.
This PR adds the instructions to install RSL RL separately from pip when
using pip installed Isaac Lab.

## Type of change

- This change requires a documentation update

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [x] 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
- [ ] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

<!--
As you go through the checklist above, you can mark something as done by
putting an x character in it

For example,
- [x] I have done this task
- [ ] I have not done this task
-->

docs/source/setup/installation/isaaclab_pip_installation.rst

@@ -67,6 +67,12 @@ To learn about how to set up your own extension project on top of Isaac Lab, vis
 
             pip install torch==2.5.1 --index-url https://download.pytorch.org/whl/cu121
 
+-  Due to limitations of PyPI, we were not able to package the RSL RL library into the Isaac Lab pip package. Therefore, we recommend first installing the RSL RL library separately
+
+   .. code-block:: bash
+
+      pip install rsl-rl@git+https://github.com/leggedrobotics/rsl_rl.git
+
 -  Before installing Isaac Lab, ensure the latest pip version is installed. To update pip, run
 
    .. tab-set::