Add tool.uv.sources to the "Settings" reference (#8543)

## Summary

Closes https://github.com/astral-sh/uv/issues/8540.

docs/reference/settings.md

@@ -198,6 +198,32 @@ package = false
 
 ---
 
+### [`sources`](#sources) {: #sources }
+
+The sources to use when resolving dependencies.
+
+`tool.uv.sources` enriches the dependency metadata with additional sources, incorporated
+during development. A dependency source can be a Git repository, a URL, a local path, or an
+alternative registry.
+
+See [Dependencies](../concepts/dependencies.md) for more.
+
+**Default value**: `"[]"`
+
+**Type**: `dict`
+
+**Example usage**:
+
+```toml title="pyproject.toml"
+
+[tool.uv.sources]
+httpx = { git = "https://github.com/encode/httpx", tag = "0.27.0" }
+pytest =  { url = "https://files.pythonhosted.org/packages/6b/77/7440a06a8ead44c7757a64362dd22df5760f9b12dc5f11b6188cd2fc27a0/pytest-8.3.3-py3-none-any.whl" }
+pydantic = { path = "/path/to/pydantic", editable = true }
+```
+
+---
+
 ### `workspace`
 
 #### [`exclude`](#workspace_exclude) {: #workspace_exclude }