Update description of github releases in docs. Resolves #3433. 

I also ended up rearranging docs/conf.py a bit.

Make tifffile (and imagecodecs) dependencies optional since the latter
is difficult to build

diff --git a/skimage/io/collection.py b/skimage/io/collection.py
index 5905ff0d0..299eb59c5 100644
--- a/skimage/io/collection.py
+++ b/skimage/io/collection.py
@@ -21,7 +21,10 @@ if LooseVersion(pil_version) < LooseVersion('7.1.0'):
          'We recommend to upgrade this library.',
          stacklevel=2)
 
-from tifffile import TiffFile
+try:
+    from tifffile import TiffFile
+except ImportError:
+    TiffFile = None
 
 
 __all__ = ['MultiImage', 'ImageCollection', 'concatenate_images',
@@ -225,6 +228,8 @@ class ImageCollection(object):
         index = []
         for fname in self._files:
             if fname.lower().endswith(('.tiff', '.tif')):
+                if TiffFile is None:
+                    raise ValueError('tifffile dependency is not available in Pyodide')
                 with open(fname, 'rb') as f:
                     img = TiffFile(f)
                     index += [(fname, i) for i in range(len(img.pages))]

Make tifffile (and imagecodecs) dependencies optional since the latter
is difficult to build

diff --git a/skimage/io/collection.py b/skimage/io/collection.py
index 5905ff0d0..299eb59c5 100644
--- a/skimage/io/collection.py
+++ b/skimage/io/collection.py
@@ -21,7 +21,10 @@ if LooseVersion(pil_version) < LooseVersion('7.1.0'):
          'We recommend to upgrade this library.',
          stacklevel=2)
 
-from tifffile import TiffFile
+try:
+    from tifffile import TiffFile
+except ImportError:
+    TiffFile = None
 
 
 __all__ = ['MultiImage', 'ImageCollection', 'concatenate_images',
@@ -225,6 +228,8 @@ class ImageCollection(object):
         index = []
         for fname in self._files:
             if fname.lower().endswith(('.tiff', '.tif')):
+                if TiffFile is None:
+                    raise ValueError('tifffile dependency is not available in Pyodide')
                 with open(fname, 'rb') as f:
                     img = TiffFile(f)
                     index += [(fname, i) for i in range(len(img.pages))]
