Fix the everest doc generator

The generator would only function within a
editable install. The behavior is changed such
that it has to be run from within the ERT
repository, in one of the parent directories of
the file that is generated.

src/everest/docs/__main__.py

@@ -1,15 +1,25 @@
+import sys
 from pathlib import Path
 
 from everest.docs.generate_docs_from_config_spec import generate_docs_pydantic_to_rst
 
-committed_file = (
-    Path(__file__).parents[3] / "docs" / "everest" / "config_generated.rst"
-).resolve()
+GENERATED_FILE = Path("docs") / "everest" / "config_generated.rst"
 
+parts = GENERATED_FILE.parts
+while parts and not Path(*parts).exists():
+    parts = parts[1:]
+if not parts:
+    print(
+        f"""The file to update ('{GENERATED_FILE}') was not found.
+The generator must be run from one of the parent directories of
+'{GENERATED_FILE.parts[-1]}' within the ERT source directory.
+Please change the current directory accordingly and re-run."""
+    )
+    sys.exit()
 
-print(f"Writing new docs contents to {committed_file}")
 docs_content = generate_docs_pydantic_to_rst()
-with open(committed_file, "w", encoding="utf-8") as fp:
+with Path(*parts).open("w", encoding="utf-8") as fp:
     fp.write(docs_content)
 
-print(f"Writing updated docs to {committed_file} complete, you may now commit it.")
+print(f"Generated docs written to {GENERATED_FILE}")
+print("Please commit the generated file.")