docs: mention where to find the code for the GitHub App

The code was moved to
https://github.com/gitgitgadget/gitgitgadget-github-app and we will want
contributors to still find it ;-)

Signed-off-by: Johannes Schindelin <[email protected]>

CONTRIBUTING.md

@@ -21,6 +21,8 @@ The most important part of the source code lives in `lib/`, and is written in Ty
 
 The exception to that rule are scripts, such as `ci-helper.ts` which backs the Azure Pipeline that is implicitly triggered via the GitHub App, which live in `scripts/`.
 
+Speaking of the GitHub App: It is implemented in a serverless fashion, as an Azure Function that gets called via webhook events, and the code backing that Azure Function lives in https://github.com/gitgitgadget/gitgitgadget-github-app. Its job is to perform very light validation of the payload and to trigger the appropriate Azure Pipeline upon receiving valid slash commands.
+
 The tests to verify that everything works as expected live in `tests/`, and use the [Jest](https://facebook.github.io/jest/) framework. Please make sure to add tests for whatever functionality you add when developing a new feature, to gain confidence that your feature or bug fix will work also in the future.
 
 If you never developed any node.js or Typescript project: you will need to get the dependencies via `npm install`, and you will want to run the tests via `npm run test`.

DESIGN.md

@@ -79,7 +79,8 @@ GitHub App and triggers an Azure Pipeline when given specific commands.
 ### Typescript
 
 A convenient way to implement a UI based on GitHub Pull Requests is to create
-a GitHub App that triggers an Azure Function that is implemented in Javascript.
+a GitHub App that triggers [an Azure Function that is implemented in
+Javascript](https://github.com/gitgitgadget/gitgitgadget-github-app).
 
 The backend is implemented as an Azure Pipeline, using Typescript to allow for
 type-safe and convenient development, and [Visual Studio