Get XMP metadata from image using Pillow. Remove ExifTool dependency

- Pillow already supports reading XMP metadata from Images
- Removes need to maintain my fork of unmaintained PyExiftool
  - This also removes dependency on system Exiftool package for
    XMP metadata extraction
- Add test to verify XMP metadata extracted from test images
- Remove references to Exiftool from Documentation

setup.py

@@ -40,6 +40,7 @@ setup(
         "aiofiles == 0.8.0",
         "dateparser == 1.1.1",
         "pyqt6 == 6.3.1",
+        "defusedxml == 0.7.1",
     ],
     include_package_data=True,
     entry_points={"console_scripts": ["khoj = src.main:run"]},