Add bilingual manual and localized docs routes

doc/manual/source/language/expression/index.md

@@ -0,0 +1,26 @@
+# Expression
+
+This chapter classifies the expressions evaluated by the language.
+
+An expression is the basic unit for representing a value, constraint, structure, or computation.
+
+```text
+Expr
+├─ literal
+├─ identifier
+├─ path reference
+├─ object
+├─ map constraint
+├─ array
+├─ array constraint
+├─ function
+├─ function call
+├─ let
+├─ match
+├─ import
+├─ composition
+├─ range refinement (`as`)
+└─ default
+```
+
+Links to each expression specification are collected in the [manual contents](../../index.md).