Reorganize to allow theme variants (#216)

## Summary

We plan to add variants of the Sphinx theme so that we can differentiate
between Core vs the Ecosystem.

They'll be distributed in the same package, and users only need to
choose the `html_theme` in their `conf.py`. They will also share a
"base" theme and override the CSS/images/HTML templates they care about.
See #215 for an
example of this.

This PR reorganizes our folder structure so that we can add variants in
followup PRs.

## Details

We call the folder `pytorch_base` because we likely will want
Pytorch-based variants for Core vs Ecosystem, and they'll use this
folder as their base. Those variants will be added in follow ups.

This also simplifies our `MANIFEST.in` and `setup.py` with
@jakelishman's recommendation from
#212 (review).
The benefit of that is we make fewer changes when adding new theme
variants (we have to add `graft qiskit_sphinx_theme/<variant>/static` to
MANIFEST.in). I verified this works by using `python -m build` and
inspecting the RECORD of the wheel, which shows the contents.

MANIFEST.in

@@ -1,5 +1,3 @@
-include qiskit_sphinx_theme/theme.conf
-include qiskit_sphinx_theme/*.html
-include qiskit_sphinx_theme/static/css/*.css
-include qiskit_sphinx_theme/static/js/*.js
-include qiskit_sphinx_theme/static/images/*.*
+recursive-include qiskit_sphinx_theme *.html
+recursive-include qiskit_sphinx_theme theme.conf
+graft qiskit_sphinx_theme/pytorch_base/static

qiskit_sphinx_theme/__init__.py

@@ -1,13 +1,17 @@
-"""Pytorch Sphinx theme.
+"""Pytorch Sphinx theme."""
 
-"""
-from os import path
+from pathlib import Path
 from warnings import warn
 
 __version__ = '1.11.0rc1'
 __version_full__ = __version__
 
 
+def _get_theme_absolute_path(folder_name: str) -> str:
+    path = Path(__file__).parent / folder_name
+    return str(path.resolve())
+
+
 def get_html_theme_path():
     """Return the absolute path to this package.
 
@@ -23,13 +27,10 @@ def get_html_theme_path():
         stacklevel=2,
         category=DeprecationWarning,
     )
-    cur_dir = path.abspath(path.dirname(path.dirname(__file__)))
-    return cur_dir
+    return _get_theme_absolute_path("pytorch_base")
 
 
 # See https://www.sphinx-doc.org/en/master/development/theming.html
 def setup(app):
-    app.add_html_theme('qiskit_sphinx_theme', path.abspath(path.dirname(__file__)))
-
-    # return explicit parallel safe
+    app.add_html_theme("qiskit_sphinx_theme", _get_theme_absolute_path("pytorch_base"))
     return {'parallel_read_safe': True, 'parallel_write_safe': True}

setup.py

@@ -1,4 +1,4 @@
-from setuptools import setup
+from setuptools import find_namespace_packages, setup
 
 setup(
     name='qiskit_sphinx_theme',
@@ -7,12 +7,7 @@
     author_email="hello@qiskit.org",
     url="https://github.com/Qiskit/qiskit_sphinx_theme",
     description="A Sphinx theme for Qiskit that is based on the Pytorch Sphinx theme.",
-    packages=[
-        "qiskit_sphinx_theme",
-        "qiskit_sphinx_theme.static.css",
-        "qiskit_sphinx_theme.static.images",
-        "qiskit_sphinx_theme.static.js",
-    ],
+    packages=find_namespace_packages(include=["qiskit_sphinx_theme*"]),
     include_package_data=True,
     entry_points={
         'sphinx.html_themes': [