Redirect sign-in and register links to a beta landing page

Public registrations on app.happydomain.org are paused. Route the
sign-in, register, and join entry points to a new /beta/ page that
explains the beta program and points users to self-hosting via the
binary or Docker image.

content/beta.en.md

@@ -0,0 +1,40 @@
++++
+title = "Join the beta"
+description = "Public registrations on app.happydomain.org are closed. Join the beta program or self-host happyDomain to get started."
++++
+
+## Online version closed
+
+Public registrations on `app.happydomain.org` are paused while we
+polish the next release. Two ways to use happyDomain right now:
+
+### 1. Join the beta program
+
+Subscribe below and we will send you an invitation once a spot opens up.
+
+<form
+  class="card card-body bg-light my-4"
+  method="post"
+  action="https://lists.happydomain.org/subscription/form"
+>
+  <input type="hidden" name="nonce" />
+  <input type="hidden" name="l" value="ef8b61ad-fa7d-4f1a-a20f-bb34ac37a3bf" />
+  <input type="hidden" name="lang" value="en" />
+  <div class="mb-3">
+    <label for="beta-email" class="form-label">Your email</label>
+    <input type="email" id="beta-email" name="email" required placeholder="j.postel@isi.edu" class="form-control" />
+  </div>
+  <altcha-widget floating class="mb-3 mx-auto" challengeurl="https://lists.happydomain.org/api/public/captcha/altcha"></altcha-widget>
+  <button type="submit" class="btn btn-primary" data-umami-event="beta-page-join">
+    Request beta access
+  </button>
+</form>
+
+### 2. Self-host happyDomain
+
+happyDomain is open source. You can run it on your own machine in minutes:
+
+- **Binary:** download a [release for your platform](/#downloads).
+- **Docker:** `docker run -p 8081:8081 happydomain/happydomain`.
+
+Source code and documentation live on [git.happydomain.org](https://git.happydomain.org/).