[Docs] Explain Matrix and sync concepts

docs/admin/01_intro.rst

+Matrix and Element
+==================
+
+`Matrix`_ is "an open network for secure, decentralized communication".
+Being an open protocol, it allows independent implementations of servers
+and clients to communicate with each other.
+Users create their account on a server they choose and are then able to
+chat with everyone in the network, just like the e-mail system works.
+
+The popular reference client `Element`_ with its team communication friendly
+user interface is a good choice for schools.
+It is available as a mobile app for Android and iOS, and as a web client,
+that can be used in a web browser.
+
+.. _Matrix: https://www.matrix.org/
+.. _Element: https://https://element.io/

docs/admin/02_concepts.rst

+Concepts of rooms and spaces
+============================
+
+In Matrix, every chat is organized as a room, where people can join
+to send and receive messages.
+Schools can map their real structure to Matrix rooms to provide each
+group or course a place to communicate.
+
+A Space is an organizational layer, that contains multiple rooms and
+can even contain other spaces. This is useful for representing child
+and parent group relationships.
+
+When the Spaces feature is enabled, AlekSIS will create a space for
+e.g. class 5a that contains the rooms 5a English, 5a Maths etc..