Build guide references nonexistent .codebase test scripts

### Description

`docs/build.md` advertises three aggregate NVIDIA test scripts that are not present in the repository:

```console
bash .codebase/scripts/nvidia/run_unittest.sh
bash .codebase/scripts/nvidia/run_e2e_test.sh
bash .codebase/scripts/nvidia/run_tutorial_test.sh
```

At current main commit `8260bc34398c2b8f36dc840fd22f741ca9294584`, no `.codebase/` path is tracked. PR #160 removed the remaining `.codebase` content, while these build instructions remained.

The commands are also published on the current live Build page: https://triton-distributed.readthedocs.io/en/latest/build.html

### Expected behavior

The build guide should direct readers to commands and paths that exist in the public checkout. Since `docs/testing.md` already documents maintained per-category unit, tutorial, and E2E commands, the stale aggregate-script sections can point readers to that guide instead of promising absent scripts.

Start with docs/build.md and compare its aggregate NVIDIA test-script references with the maintained commands in docs/testing.md. Update the build guide so it points to paths and commands present in the public checkout, then verify no nonexistent .codebase script references remain.

Written by the indexing model from the issue text.