Fill in the empty readmes

Affine, Overleaf, and the registry had empty readmes, which the layout check now rejects.

affine/readme.md

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+# Affine
+
+Runs [Affine](https://affine.pro) on `monoco` as Docker containers, with its
+state in the shared Postgres and Valkey units.
+
+## Variables
+
+- `host`: public hostname, `wiki.tanshu.com`.
+- `revision`: image tag, `stable` by default.
+- `db_password`: password for the `affine` database.
+
+Set them in [vars/default.yml](vars/default.yml).
+
+## Run
+
+    ansible-playbook playbook.yml
+
+The `affine-migration` role is commented out. Enable it for the first run and
+after a major version upgrade.