Enable shorter gallery URLs (#475)

doc/conf.py

@@ -174,8 +174,22 @@ def gallery_spec(name):
     'sections': [gallery_spec(project) for project in projects],
 }
 
-def to_gallery_redirects():
-    # Redirects from /projname to /gallery/projname/<index>
+def root_and_gallery_index_redirects():
+    """
+    The projects were originally (pyviz era) at the root, e.g. examples.pyviz.org/attractors/attractors.html
+    At the time, redirects were implemented to allow shorter URLs like examples.pyviz.org/attractors(/).
+    Projects are now under the /gallery folder, because otherwise they would all
+    end up being displayed in the navbar, as the PyData Sphinx Theme displays the
+    top-level toctree in the navbar. The first goal of this function is to
+    generate these redirects to preserve these old links:
+    examples.holoviz.org/attractors(/) -> examples.holoviz.org/gallery/attractors/attractors.html
+
+    For mono-notebook projects (the majority), links like examples.pyviz.org/gallery/boids(/)
+    don't redirect by default to examples.pyviz.org/gallery/boids/boids.html.
+    The second goal of the function is to enable this by creating this redirect link:
+    examples.holoviz.org/gallery/boids/index -> examples.holoviz.org/gallery/boids/boids
+    Which whill enable examples.holoviz.org/gallery/boids(/)
+    """
     redirects = {}
     for project in projects:
         notebooks = find_notebooks(project, root='..')
@@ -186,7 +200,10 @@ def to_gallery_redirects():
             index = nbstems[0]
         else:
             raise RuntimeError(f'Too many notebooks found: {nbstems}')
-        redirects[project] = f'gallery/{project}/{index}'
+        redirects[f'{project}/index'] = f'gallery/{project}/{index}'
+        if index != 'index':
+            # Projects with an index notebook don't need the redirect
+            redirects[f'gallery/{project}/index'] = f'gallery/{project}/{index}'
     return redirects
 
 top_level_redirects = {
@@ -252,7 +269,8 @@ def to_gallery_redirects():
     **top_level_redirects,
     **project_direct_links,
     # Links from e.g. /attractors to /gallery/attractors/index.html
-    **to_gallery_redirects(),
+    # And from e.g. /gallery/boids/index.html to /gallery/boids/boids.html
+    **root_and_gallery_index_redirects(),
 }
 
 html_context.update({