feat(docs): start configuration page

docs/content/docs/configuration.md

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+# Configuration
+
+All configuration variables can be provided either through the configuration
+file, as environment variables or using CLI flags. All environment variable
+names are derived from their place in the TOML file, prepended with `OTTER_`.
+
+Variables are grouped by their section in the configuration file, e.g. the
+variables under the `net` section should be placed inside the `[net]` group in
+the TOML file.
+
+## Top-level settings
+
+* `data_dir` (`OTTER_DATA_DIR`): directory where Otter stores its data. This
+  directory must exist and be accessible by the server when starting up.
+    * Default: `./data`
+* `session_cleanup_interval` (`OTTER_SESSION_CLEANUP_INTERVAL`): how frequently
+  (in seconds) the session cleanup background job should be run. This job
+  removes expired user sessions from the database.
+    * Default: `86400` (once a day)
+* `log_level` (`OTTER_LOG_LEVEL`): how verbose the logging should be; one of
+  `debug`, `info`, `warn` or `error`
+    * Default: `warn`
+
+## Network (`net`)
+
+* `type` (`OTTER_NET_TYPE`): type of network connection to establish; one of
+  `tcp`, `unix`
+    * Default: `tcp`
+* `domain` (`OTTER_NET_DOMAIN`): domain to bind TCP socket to; only applicable
+  when `net.type` is `tcp`.
+    * Default: `127.0.0.1`
+* `port` (`OTTER_NET_PORT`): port to bind TCP socket to; only applicable when
+  `net.type` is `tcp`.
+    * Default: `8080`
+* `path` (`OTTER_NET_PATH`): path to bind Unix socket to; only applicable when
+  `net.type` is `unix`.