Migrate to Docusaurus (#329)

* docs: initial migration to docusaurus

* website: add custom font, update blurbs and icons

* website: update splash

* root: update links to docs

* flows: use .pbflow extension so docusaurus doesn't mangle the files

* e2e: workaround prospector

* Squashed commit of the following:

commit 1248585dca
Author: Jens Langhammer <jens.langhammer@beryju.org>
Date:   Sun Nov 15 20:46:53 2020 +0100

    e2e: attempt to fix prospector error again

commit 1319c480c4
Author: Jens Langhammer <jens.langhammer@beryju.org>
Date:   Sun Nov 15 20:41:35 2020 +0100

    ci: install previous python version for upgrade testing

* web: update accent colours and format

* website: format markdown files

* website: fix colours for text

* website: switch to temporary accent colour to improve readability

* flows: fix path for TestTransferDocs

* flows: fix formatting of tests

website/docs/installation/reverse-proxy.md

@@ -0,0 +1,45 @@
+---
+title: passbook behind a reverse-proxy
+---
+
+If you want to access passbook behind a reverse-proxy, use a config like this. It is important that Websocket is enabled, so that Outposts can connect.
+
+```
+map $http_upgrade $connection_upgrade {
+    default upgrade;
+    ''      close;
+}
+
+server {
+    # Server config
+    listen 80;
+    server_name sso.domain.tld;
+
+    # 301 to SSL
+    location / {
+            return 301 https://$host$request_uri;
+    }
+}
+server {
+    # Server config
+    listen 443 ssl http2;
+    server_name sso.domain.tld;
+
+    # SSL Certs
+    ssl_certificate /etc/letsencrypt/live/domain.tld/fullchain.pem;
+    ssl_certificate_key /etc/letsencrypt/live/domain.tld/privkey.pem;
+
+    # Proxy site
+    location / {
+        proxy_pass https://<hostname of your passbook server>;
+        proxy_http_version 1.1;
+        proxy_set_header X-Forwarded-Proto https;
+        proxy_set_header X-Forwarded-Port 443;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        # This needs to be set inside the location block, very important.
+        proxy_set_header Host $host;
+        proxy_set_header Upgrade $http_upgrade;
+        proxy_set_header Connection $connection_upgrade;
+    }
+}
+```