docs: spec language reference page + writing-specs rewrite (#46)

* docs(architecture): add device/emulator node and XCTest edge to diagram

* docs(manual): rewrite writing-specs with accurate API and updated examples

* docs(manual): add spec-language reference page

* docs(sidebar): add spec-language entry to sidebar nav

docs/manual/index.md

@@ -6,5 +6,6 @@ title: Manual
 
 - [Getting started](./getting-started/)
 - [Writing specs](./writing-specs/)
+- [Spec language reference](./spec-language/)
 - [Runs](./runs/)
 - [CLI reference](./cli/)