docs: create a dedicated page for the NG0203 error (#46166)

Adds a dedicated error page for NG0203 (injection context).

PR Close #46166

aio/content/errors/NG0203.md

@@ -0,0 +1,83 @@
+@name `inject()` must be called from an injection context
+@category runtime
+@shortDescription `inject()` must be called from an injection context (a constructor, a factory function or a field initializer)
+
+@description
+You see this error when you try to use the `inject()` function outside of the allowed injection context. The injection context is available during the class creation and initialization. 
+
+In practice the `inject()` calls are allowed in a constructor, a constructor parameter and a field initializer:
+
+```typescript
+@Injectable({providedIn: 'root'})
+export class Car {
+  radio: Radio|undefined;
+
+  // OK: field initializer
+  spareTyre = inject(Tyre);
+  
+  // OK: constructor parameter
+  constructor(readonly engine: Engine = inject(Engine)) {
+
+    // OK: constructor body
+    this.radio = inject(Radio);
+  }
+}
+```
+
+It is also legal to call `inject` from a provider's factory:
+
+```typescript
+providers: [
+  {provide: Car, useFactory: () => {
+    // OK: a class factory
+    const engine = inject(Engine);
+    return new Car(engine);
+  }}
+]
+```
+
+The `inject()` calls might be wrapped in an intermediate function and will work correctly as long as a wrapper function is used in one of the allowed contexts:
+
+```typescript
+function getAndLog<T>(token: ProviderToken<T>): T {
+  const instance = inject(token);
+
+  console.log('Getting', token.toString());
+
+  return instance;
+}
+
+@Injectable({providedIn: 'root'})
+export class Car {
+  constructor(readonly engine: Engine = getAndLog(Engine)) {
+  }
+}
+```
+
+Calls to the `inject()` function outside of the class creation context will result in error. Most notably, calls to `inject()` are disallowed after a class instance was created, in methods:
+
+```typescript
+@Injectable({providedIn: 'root'})
+export class Car {
+
+  start() {
+    // ERROR: too late, the Car instance was already created and fully initialized
+    const engine = inject(Engine);
+    engine.start();
+  }
+}
+```
+
+@debugging
+
+Work backwards from the stack trace of the error to identify a place where the disallowed call to `inject()` is located. 
+
+To fix the error move the `inject()` call to an allowed place (usually a class constructor or a field initializer).
+
+<!-- links -->
+
+<!-- external links -->
+
+<!-- end links -->
+
+@reviewed 2022-05-27