docs: add docs for outposts

docs/outposts/outposts.md

@@ -0,0 +1,26 @@
+# Outposts
+
+An outpost is a single deployment of a passbook component, which can be deployed in a completely separate environment. Currently, only the Proxy Provider is supported as outpost.
+
+![](outposts.png)
+
+Upon creation, a service account and a token is generated. The service account only has permissions to read the outpost and provider configuration. This token is used by the Outpost to connect to passbook.
+
+To deploy an outpost, you can for example use this docker-compose snippet:
+
+```yaml
+version: 3.5
+
+services:
+  passbook_proxy:
+    image: beryju/passbook-proxy:0.10.0-stable
+    ports:
+      - 4180:4180
+      - 4443:4443
+    environment:
+      PASSBOOK_HOST: https://your-passbook.tld
+      PASSBOOK_INSECURE: 'true'
+      PASSBOOK_TOKEN: token-generated-by-passbook
+```
+
+In future versions, this snippet will be automatically generated. You will also be able to deploy an outpost directly into a kubernetes cluster.w

docs/providers/proxy.md

@@ -1,5 +1,8 @@
 # Proxy Provider
 
+!!! info
+    This provider is to be used in conjunction with [Outposts](../outposts/outposts.md)
+
 This provider protects applications, which have no built-in support for OAuth2 or SAML. This is done by running a lightweight Reverse Proxy in front of the application, which authenticates the requests.
 
 passbook Proxy is based on [oauth2_proxy](https://github.com/oauth2-proxy/oauth2-proxy), but has been integrated more tightly with passbook.

docs/upgrading/to-0.10.md

@@ -16,6 +16,8 @@ This update brings a lot of big features, such as:
 
   This is a new Object type, used currently only by the Proxy Provider. It manages the creation and permissions of service accounts, which are used by the outposts to communicate with passbook.
 
+  See [Outposts](../outposts/outposts.md)
+
 - Flow Import/Export
 
   Flows can now be imported and exported. This feature can be used as a backup system, or to share complex flows with other people. Example flows have also been added to the documentation to help you get going with passbook.