docs: add Fern documentation infrastructure (#2053)

Add Fern infrastructure for NIXL documentation:
- Configuration for deployment to `docs.nvidia.com/nixl`
- NVIDIA styling and assets
- Documentation sources and generation scripts
- Fern validation CI (checks for valid links, valid mdx formatting etc.)

Just two placeholder pages to verify the navigation in preview.  
Eventually land NIXL documentation on docs.nvidia.com, similar to the
[NVIDIA Dynamo Documentation](https://docs.nvidia.com/dynamo/)

Signed-off-by: Lukas Humbel <lhumbel@nvidia.com>