docs(git): add subdirectory example (#10894)
## Summary
Closes #10889.
As kinda hinted in the issue, would adding a `--subdirectory` flag to
`uv add` make sense? This would IMO be more discoverable.
## Test Plan
Ran the command added to the documentation:
```console
$ uv add git+https://github.com/langchain-ai/langchain#subdirectory=libs/langchain
Resolved 40 packages in 203ms
Built langchain @ git+https://github.com/langchain-ai/langchain@f2ea62f63209130bfc56b1fe7d0fa7c299bbf352#subdirectory=libs/langchain
Prepared 19 packages in 837ms
Installed 37 packages in 14ms
[...]
```
