Add deployment notes

docs/deploy.md

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+# Deploying the web apps
+
+The various web apps (ente Photos, ente Auth) are deployed on Cloudflare Pages.
+They also use Cloudflare Workers for some tasks.
+
+The web app repository deploys multiple different apps (the photos app, the auth
+app). Some of them get deployed to multiple different endpoints (e.g. the main
+branch of photos app gets deployed to testing.ente.io, the while the
+photos-release branch is the production deployment).
+
+The apps are under the app directory:
+
+- photos - The ente Photos app
+- auth - The Authenticator app
+- cast - The cast app, which can be thought of as an independent subset of
+  Photos app functionality.
+- ... and more in the future.
+
+We have configured the GitHub integration provided by Cloudflare Pages app on
+the photos-web repository. This integration watches for pushes to all branches.
+In all cases, it runs the same script, `scripts/deploy.sh`.
+
+Internally it uses the `CF_PAGES_BRANCH` environment variable to decide what
+exactly to build ([CF
+docs](https://developers.cloudflare.com/pages/how-to/build-commands-branches/)).
+
+Then, for some special branches, we have configured CNAME aliases (Cloudflare
+calls them Custom Domains) to give a stable URL to some of these deployments
+Here is a potentially out of date list of CNAMEs and the corresponding branch;
+see the Cloudflare dashboard for the latest:
+
+- _testing.ente.io_: `main`
+- _web.ente.io_: `photos-release`
+- _auth.ente.io_: `auth-release`
+
+Thus to trigger a, say, production deployment of the photos app, we can open and
+merge a PR into the `photos-release` branch. Cloudflare will then build and
+deploy the code to _web.ente.io_.
+
+## Adding a new app
+
+1. Add a mapping in `scripts/deploy.sh` (in the [photos-web
+   repository](https://github.com/ente-io/photos-web)).
+
+2. Add a [Custom Domain in
+   Cloudflare](https://developers.cloudflare.com/pages/how-to/custom-branch-aliases/)
+   pointing to this branch's deployment.