doc: add docs on new minimal theme option

docs/source/index.md

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 install
 syntax
+options
 testing
 zreferences
 ```

docs/source/options.md

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+# Options
+
+This package has the option to choose a more **minimal** color scheme.
+
+The aim is to create admonitions that are clearly different to the core text with
+colors that do not over emphasises the admonition.
+
+To enable the `minimal` color scheme you can use the following.
+
+## Jupyter Book Project
+
+Add `proof_minimal_theme = True` to your `_config.yml`
+
+```yaml
+sphinx:
+  config:
+    proof_minimal_theme: true
+```
+
+## Sphinx Project
+
+Add `proof_minimal_theme = True` to your `conf.py`