Replace the doc theme with the pydoctheme

Released under the Python Software Foundation License, this one
doesn't look awful (no green top bar, huzzah!).

Lightly forked it to remove the donation footer (we don't take any)
and to change the python references to fish references.

The image is just our favicon, which is a stylized "f" and therefore
not fantastic (are we facebook?), but it's the best I found, and the
thing before had no images at all.

Fixes #6500
(as far as I'm concerned)

doc_src/python_docs_theme/__init__.py

@@ -0,0 +1,12 @@
+import os
+
+
+def setup(app):
+    current_dir = os.path.abspath(os.path.dirname(__file__))
+    app.add_html_theme(
+        'python_docs_theme', current_dir)
+
+    return {
+        'parallel_read_safe': True,
+        'parallel_write_safe': True,
+    }