Add initial standalone mode docs

Author: tulir

maubot/SUMMARY.md

@@ -11,6 +11,7 @@
   - [mbc auth](./usage/cli/auth.md)
   - [mbc build](./usage/cli/build.md)
 - [Management API](./management-api.md)
+- [Standalone mode](./usage/standalone.md)
 
 # Plugin development
 - [Getting started](./dev/getting-started.md)

maubot/usage/standalone.md

@@ -0,0 +1,33 @@
+# Standalone mode
+
+The normal mode in maubot is very dynamic: the config file doesn't really
+contain any runtime details other than a general web server, database, etc.
+Everything else is set up at runtime using the web management interface or
+the [management API](../management-api.md) directly. This dynamicness is very
+useful for developing bots and works fine for deploying it on personal servers,
+but it's not optimal for larger production deployments.
+
+The solution is standalone mode: a separate entrypoint that runs a single maubot
+plugin with a predefined Matrix account.
+
+## Basic usage
+
+0. Set up a virtual environment.
+  1. Create with `virtualenv -p /usr/bin/python3 .venv`
+  2. Activate with `source .venv/bin/activate`
+1. Install maubot into the virtualenv with `pip install --upgrade maubot[all]`
+  * `[all]` at the end will install all optional dependencies!
+    The e2ee optional dependency requires libolm3 to be installed natively.
+  * Alternatively install the latest git version with
+    `pip install --upgrade git+https://github.com/maubot/maubot.git#egg=maubot[all]`
+2. Extract the plugin you want to run into the directory
+   (at least `maubot.yaml` and some Python modules should be there).
+3. Install any dependencies that the plugin has into the virtualenv manually
+   (they should be listed in `maubot.yaml`).
+4. Copy the [standalone example config] to the same directory and fill it out.
+  * If the plugin has a config, you should copy the contents from the plugin's
+    `base-config.yaml` into the `plugin_config` object in the standalone config,
+    then fill it out as needed.
+5. Run the bot with `python -m maubot.standalone`
+
+[standalone example config]: https://github.com/maubot/maubot/blob/master/maubot/standalone/example-config.yaml