ci: permit manual deploys from main

We hadn't yet merged the monorepo PR that deactivates the old deploy [0]
logic, so the two fought and the old mdbook site was deployed for a
while. Let's add a config line so we can deploy directly from the web
UI, and not have to push triggering commits, e.g. [1].

[0] penumbra-zone/penumbra#4751
[1] b329c69

Author: conorsch

.github/workflows/firebase-hosting-merge.yml

@@ -5,6 +5,8 @@
 
 name: Deploy to Firebase Hosting on merge
 on:
+  # Permit manual deploys via GHA web UI.
+  workflow_dispatch:
   push:
     branches:
       - main