Include _static/images/logo.png so projects can use it instead of c…

…opying `no_image.png` (#318)

Part of #262.

14 projects are copying around the same `no_image.png` file. It's
helpful so that you can set `nbsphinx_thumbnails` to point to it. This
PR adds the file to `qiskit_sphinx_theme` so that they can instead use
the standard file if they want.

They don't have to switch to this file. And repos can also still
override `images/logo.png` how they want, e.g. that Metal has it be a
Metal specific logo currently. Their override will take precedence.

--

Note that we already distribute `static/images/logo.svg`. But
`nbsphinx_thumbnails` doesn't work with an SVG.

--

This PR also removes `gallery.css` from the example docs. We don't use
`sphinx-gallery`, so this was a bad copy and paste.

example_docs/docs/conf.py

@@ -25,7 +25,6 @@
 # This allows including custom CSS and HTML templates.
 html_static_path = ['_static']
 templates_path = ['_templates']
-html_css_files = ['gallery.css']
 
 # Sphinx should ignore these patterns when building.
 exclude_patterns = ['_build', '**.ipynb_checkpoints']
@@ -101,5 +100,6 @@
 # Settings for Jupyter notebooks.
 nbsphinx_execute = "never"
 nbsphinx_thumbnails = {
-    "sphinx_guide/notebook": "_static/no_image.png",
+    # Default image for thumbnails.
+    "**": "_static/images/logo.png",
 }