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/property-mappings/expression.md

@@ -0,0 +1,15 @@
+---
+title: Property Mapping Expressions
+---
+
+The property mapping should return a value that is expected by the Provider/Source. Supported types are documented in the individual Provider/Source. Returning `None` is always accepted and would simply skip the mapping for which `None` was returned.
+
+:::note
+These variables are available in addition to the common variables/functions defined in [**Expressions**](../expressions/index.md)
+:::
+
+### Context Variables
+
+-   `user`: The current user. This may be `None` if there is no contextual user. ([ref](../expressions/reference/user-object.md))
+-   `request`: The current request. This may be `None` if there is no contextual request. ([ref](https://docs.djangoproject.com/en/3.0/ref/request-response/#httprequest-objects))
+-   Other arbitrary arguments given by the provider, this is documented on the Provider/Source.