[Docs] Use Sphinx-immaterial-igwn

Use sphinx-immaterial-igwn theme for docs

• Adds theme (and html_options) for sphinx-immaterial-igwn to docs/conf.py
• Removes other config options from the previous theme.
• Removes _static/gwosc.css
• Adds sphinx_immaterial_igwn dependency to pyproject.toml [docs] table.