[Syntax lookup] Add doc for explicit string interpolation (#404)

* Add documentation for explicit string interpolation

* Apply review comments

Author: kellydanma

misc_docs/syntax/language_string_interpolation.mdx

@@ -0,0 +1,34 @@
+---
+id: "string-interpolation"
+keywords: ["string", "interpolation"]
+name: "``"
+summary: "This is the `string interpolation` syntax."
+category: "languageconstructs"
+---
+
+A `string interpolation` is composed of two backticks. It allows Unicode characters and embeddable values and expressions with the `${myValue}` syntax.
+
+### Example
+
+<CodeTab labels={["ReScript", "JS Output"]}>
+
+```res
+let language = "ReScript"
+
+let exclamation = `${language} is pretty cool.
+Right?
+🎉
+`
+```
+
+```js
+var language = "ReScript";
+
+var exclamation = language + " is pretty cool.\nRight?\n🎉\n";
+```
+
+</CodeTab>
+
+### References
+
+* [String interpolation](/docs/manual/latest/primitive-types#string-interpolation)