Adds Isaac Lab Reference Architecture documentation (#1371)

# Description

Adding Isaac Lab Reference Architecture documentation to present an overview of the end-to-end robot learning process with Isaac Lab.

## 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
- [x] 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
-->

---------
Co-authored-by: Kelly Guo <kellyg@nvidia.com>

docs/index.rst

@@ -127,6 +127,7 @@ Table of Contents
    :maxdepth: 1
    :caption: References
 
+   source/refs/reference_architecture/index
    source/refs/additional_resources
    source/refs/contributing
    source/refs/troubleshooting