Add Rhai book.

doc/src/engine/expressions.md

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+Evaluate Expressions Only
+========================
+
+{{#include ../links.md}}
+
+Sometimes a use case does not require a full-blown scripting _language_, but only needs to evaluate _expressions_.
+
+In these cases, use the `Engine::compile_expression` and `Engine::eval_expression` methods or their `_with_scope` variants.
+
+```rust
+let result = engine.eval_expression::<i64>("2 + (10 + 10) * 2")?;
+```
+
+When evaluating _expressions_, no full-blown statement (e.g. `if`, `while`, `for`) - not even variable assignment -
+is supported and will be considered parse errors when encountered.
+
+```rust
+// The following are all syntax errors because the script is not an expression.
+
+engine.eval_expression::<()>("x = 42")?;
+
+let ast = engine.compile_expression("let x = 42")?;
+
+let result = engine.eval_expression_with_scope::<i64>(&mut scope, "if x { 42 } else { 123 }")?;
+```