docs: organise documentation

docs/services/stalwart.md

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+### stalwart
+
+**initial setup**
+
+1. server identity
+    - server hostname: `post.atom.town`
+    - default email domain: `atom.town`
+    - automatically obtain tls certificate: `true`
+    - generate email signing keys: `true`
+2. storage
+    - main data storage: `rocksdb`
+    - path: `/var/lib/stalwart/`
+3. account directory
+    - directory type: `use internal`
+4. logging
+    - log destination: `console`
+5. automatic dns management
+    - dns server type: `porkbun`
+    - description: `porkbun`
+
+**further setup**
+
+- settings -> security -> allowed ips
+  - added `172.18.0.0/16`; reason: `docker internal`
+- settings -> network -> http -> security
+  - permissive cors policy: `true`   **[restart needed]**
+- settings -> authentication -> oidc provider
+  - oauth settings
+    - key: `secret read from environment variable`
+    - variable name: `OIDC_KEY`
+  - openid connect
+    - signature algorithm: `ECDSA using P-384 and SHA-384`
+    - signature key: `secret read from environment variable`
+    - variable name: `OIDC_SIGNATURE_KEY`
+- management -> directory -> oauth clients
+  - create oauth client
+    - client id: `gitea`
+    - description: `Gitea`
+    - client secret: `<generate_secret>`
+    - redirect uris: `https://git.atom.town/user/oauth2/Stalwart/callback`
+
+**useful links**
+
+- https://stalw.art/docs
+- https://testconnectivity.microsoft.com/tests/Imap/input
+- https://mail-tester.com
+</details>