Add "Asynchronous" term to glossary (#7100)

feat(glossary): Add "Asynchronous" entry with Dart-specific context

- Added comprehensive entry explaining Dart's event loop model, Futures,
  async/await, Streams, and isolates
- Short description optimized for site-wide hover tooltips
- Related links to official Dart async programming docs
- Labels for filtering: async, futures, event-loop, concurrency

Enables better developer onboarding by defining core Dart concurrency 
concepts in the shared glossary system.

Contributes to #6461

---------

Co-authored-by: Parker Lougheed <parlough@gmail.com>

Author: Anushreebasics

site/lib/src/style_hash.dart

@@ -2,4 +2,4 @@
 // dart format off
 
 /// The generated hash of the `main.css` file.
-const generatedStylesHash = 'RHmO6DK8CuPj';
+const generatedStylesHash = '';

src/data/glossary.yml

@@ -50,6 +50,51 @@
   alternate:
     - "Quick assist"
 
+- term: "Asynchronous"
+  short_description: |-
+    Programming paradigm in Dart where operations like
+    I/O or network calls complete later without blocking the event loop.
+  long_description: |-
+    Dart uses a single-threaded event loop model for asynchronous programming, 
+    allowing non-blocking execution of time-consuming tasks such as
+    file I/O, HTTP requests, and timers.
+    Unlike synchronous code that blocks until completion, 
+    async operations queue through microtasks/events, keeping UIs responsive.
+
+    Key components include:
+
+    - **Futures**:
+      Represent a value available later, created by APIs like `http.get()`.
+    - **async/await**:
+      Dart keywords for writing maintainable and readable asynchronous code.
+    - **Streams**:
+      Handle sequences of async events like data from web sockets.
+    - **Isolates**:
+      Enable true parallelism through
+      independent execution contexts with message passing.
+  related_links:
+    - text: "Asynchronous programming: futures, async, await"
+      link: "/libraries/async/async-await"
+      type: "doc"
+    - text: "Asynchronous programming overview"
+      link: "/language/async"
+      type: "doc"
+    - text: "dart:async library"
+      link: "https://api.dart.dev/dart-async"
+      type: "api"
+    - text: "Future class reference"
+      link: "https://api.dart.dev/dart-async/Future-class.html"
+      type: "api"
+  labels:
+    - "async"
+    - "futures"
+    - "event-loop"
+    - "concurrency"
+  alternate:
+    - "Async"
+    - "Non-blocking"
+
+
 - term: "Bottom type"
   short_description: |-
     A type that has no values and is a subtype of all other types.