Make doc prettier

[pllim]

• Move the RST files up a level and delete docs/spherical_geometry.
• Move the contents from README.rst into a proper content in index.rst in the docs. And then just link to ReadTheDocs in the README. Then you can delete the duplicate inside.png in the root dir (needed it for README.rst images not showing #69 and added in Modernization galore #243).
• Use a more modern theme, preferably with dark mode. FWIW astropy now uses pydata-sphinx-theme.