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/_template/page.html

@@ -34,6 +34,7 @@
       <ul>
         <li><a href="__ROOT__manual/getting-started/">Getting started</a></li>
         <li><a href="__ROOT__manual/writing-specs/">Writing specs</a></li>
+        <li><a href="__ROOT__manual/spec-language/">Spec language reference</a></li>
         <li><a href="__ROOT__manual/runs/">Runs</a></li>
         <li><a href="__ROOT__manual/inspect/">Inspect</a></li>
         <li><a href="__ROOT__manual/cli/">CLI reference</a></li>