docs(python): add link to requirements file reference (#925)

Author: hknutsen

docs/workflows/python.md

@@ -111,7 +111,7 @@ The path, relative to the working directory, to create a virtual Python environm
 
 ### (*Optional*) `requirements`
 
-A [requirement specifier](https://pip.pypa.io/en/stable/reference/requirement-specifiers/) or the path, relative to the working directory, of a requirements file (usually `requirements.txt`) that specifies Python dependencies to install.
+A [requirement specifier](https://pip.pypa.io/en/stable/reference/requirement-specifiers/) or the path, relative to the working directory, of a [requirements file](https://pip.pypa.io/en/stable/reference/requirements-file-format/) (usually `requirements.txt`) that specifies Python dependencies to install.
 
 For example, set the value to `"."` to install dependencies from a `pyproject.toml` file in the working directory.