doc: Add _pkgdown.yml

_pkgdown.yml

+url: ~
+template:
+  bootstrap: 5
+  bslib:
+    base_font: {google: "Atkinson Hyperlegible"}
+    code_font: {google: "JetBrains Mono"}
+  theme: a11y-light
+
+reference:
+  - title: "🟠 **`ChemoSpec`**"
+    contents:
+    #- has_keyword("from_ChemoSpec")
+    - to_ChemoSpec
+    - has_keyword("to_ChemoSpec")
+  - title: "🔵 **`hyperSpec`**"
+    contents:
+    #- has_keyword("from_hyperSpec")
+    - to_hyperSpec
+    - has_keyword("to_hyperSpec")