docs: organise documentation

docs/services/bulwark.md

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+### bulwark
+
+> [!NOTE]
+> **todo** // detail setup
+
+**useful links**
+
+- https://bulwarkmail.org/docs

docs/services/caddy.md

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+### caddy
+
+> [!NOTE]
+> **todo** // detail setup

docs/services/gitea.md

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+### gitea
+
+**initial setup**
+
+- database settings
+  - database type: `sqlite3`
+  - path: `/data/gitea/gitea.db`
+- general settings
+  - site title: `atom.town`
+  - server domain: `git.atom.town`
+  - ssh server port: `22`
+  - gitea base url: `https://git.atom.town/`
+  - enable update checker: `true`
+- email settings
+  - smtp host: `post.atom.town`
+  - smtp port: `465`
+  - send email as: `"Gitea" <system@atom.town>`
+  - smtp username: `system@atom.town`
+  - smtp password: `<password>`
+  - enable email notifications: `true`
+- server and third-party service settings
+  - enable openid sign-in: `false`
+  - disable self-registration: `false`
+  - allow registration only through external services: `true`
+  - enable openid self-registration: `true`
+  - hidden email domain: `git.atom.town`
+  - password hash algorithm: `argon2`
+</details>
+
+**further setup**
+
+- admin settings -> identity & access -> authentication sources
+  - add authentication source
+    - authentication type: `OAuth2`
+    - authentication name: `Stalwart`
+    - oauth2 provider: `OpenID Connect`
+    - client id (key): `gitea`
+    - client secret: `<secret_from_stalwart>`
+    - openid connect auto discovery url: `https://post.atom.town/.well-known/openid-configuration`

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>