docs: organise documentation

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`