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/src/css/custom.css

@@ -0,0 +1,47 @@
+@font-face {
+    font-family: "DIN 1451 Std";
+    src: url("/fonts/DINEngschriftStd.woff2") format("woff2"),
+        url("/fonts/DINEngschriftStd.woff") format("woff");
+    font-weight: normal;
+    font-style: normal;
+    font-display: swap;
+}
+
+:root {
+    --ifm-color-primary: #3578e5;
+    --ifm-color-primary-dark: #1d68e1;
+    --ifm-color-primary-darker: #1b62d4;
+    --ifm-color-primary-darkest: #1751af;
+    --ifm-color-primary-light: #4e89e8;
+    --ifm-color-primary-lighter: #5a91ea;
+    --ifm-color-primary-lightest: #80aaef;
+}
+
+.button.button--outline {
+    color: var(--ifm-navbar-link-color) !important;
+}
+
+.navbar {
+    background-color: var(--ifm-color-primary);
+    box-shadow: none;
+}
+
+.navbar__title {
+    font-family: "DIN 1451 Std";
+    font-size: 2em;
+    margin-top: 2px;
+}
+
+.hero--primary {
+    -webkit-clip-path: polygon(0 0, 100% 0, 100% 100%, 0 calc(100% - 3vw));
+    clip-path: polygon(0 0, 100% 0, 100% 100%, 0 calc(100% - 3vw));
+}
+
+.hero_image {
+    flex-direction: column;
+    justify-items: center;
+}
+
+.hero_image > img {
+    max-height: 200px;
+}