Point license links in docs to OpenSlide website

It provides better UX than a GitHub object link.

Author: bgilbert

README.md

@@ -63,4 +63,4 @@ components so OpenSlide Python can find OpenSlide, see the package
 ## License
 
 OpenSlide Python is released under the terms of the [GNU Lesser General
-Public License, version 2.1](https://raw.github.com/openslide/openslide-python/main/lgpl-2.1.txt).
+Public License, version 2.1](https://openslide.org/license/).

doc/index.rst

@@ -38,7 +38,7 @@ Public License, version 2.1`_.
 .. _Trestle: https://openslide.org/formats/trestle/
 .. _Ventana: https://openslide.org/formats/ventana/
 .. _`Generic tiled TIFF`: https://openslide.org/formats/generic-tiff/
-.. _`GNU Lesser General Public License, version 2.1`: https://raw.github.com/openslide/openslide-python/main/lgpl-2.1.txt
+.. _`GNU Lesser General Public License, version 2.1`: https://openslide.org/license/
 
 
 Installing