add SimplcityHL book content

Author: imaginator

mkdocs.yml

@@ -5,8 +5,8 @@ plugins:
 theme:
   name: material
   font:
-    text: Roboto
-    code: Roboto Mono
+    text: Open Sans
+    code: Source Code Pro
   favicon: assets/logo-symbol.svg
   logo: assets/logo-symbol.svg
   features:
@@ -139,6 +139,15 @@ nav:
       - Simplicity for EVM developers: documentation/simplicity-for-evm-developers.md
       - How Simplicity Works: documentation/how-simplicity-works.md
       - Simplicity Compared: documentation/simplicity-compared.md
+      - SimplicityHL Reference:
+          - Overview: documentation/simplicityhl/overview.md
+          - Types and Values: documentation/simplicityhl/type.md
+          - Type Aliases: documentation/simplicityhl/type_alias.md
+          - Type Casting: documentation/simplicityhl/type_casting.md
+          - Let Statements: documentation/simplicityhl/let_statement.md
+          - Match Expression: documentation/simplicityhl/match_expression.md
+          - Functions: documentation/simplicityhl/function.md
+          - Programs: documentation/simplicityhl/program.md
   - Roadmap: roadmap.md
   - FAQ: faq.md
-  - Contibute: contribute.md
+  - Contribute: contribute.md