docs: note on intalling GPU toolkits for contributors

[nuke-web3]

Toolchain installed on Ubuntu working with command cargo run -r --features cuda serve --model TabbyML/StarCoder-1B --device cuda.

For me I needed to set https://getdocs.org/Cmake/docs/3.21/envvar/cudacxx in your environment (~/.bashrc for me adding export CUDACXX=/usr/local/cuda/bin/nvcc after install of CUDA)

Closes #2033

[nuke-web3]

I can run the service fine but get

DEBUG tabby::services::code: crates/tabby/src/services/code.rs:61: Source code index is not ready `Failed to open the directory: 'DoesNotExist("~/.tabby/index")'`

Is there some manual generation of that index needed on first startup not mentioned in CONTRIBUTING.md?

[wsxiaoys]

This is a debug warning, you can safely ignore it. To config and populate the index, use Git Providers manual in admin dashboard.

[nuke-web3]

Got it working with admin panel http://localhost:8080/swagger-ui/#/v1/completion tested with "Try it out" to give something sensible 🎉