Add PWA-related instructions and information

docs/user/20_pwa.rst

+PWA (progressive web application)
+=================================
+
+What is a progressive web application?
+--------------------------------------
+
+A PWA is an application developed with common web technologies and
+delivered in form of a website, but which offers some features a
+traditional website does not and ,overall, creates an impression that
+resembles that of a native application.
+
+AlekSIS PWA features
+--------------------
+
+The AlekSIS PWA offers the following features (not all available on all
+platforms):
+
+-  Installable and displayable in a separate window
+-  Caching and serving, if given page cannot be accessed, of
+   non-interactive pages and needed assets
+-  Provision of an offline fallback page if wanted page cannot be
+   accessed and there is no cached one
+-  Indicator whether the served page is served from the PWA cache
+
+Installation of the PWA
+-----------------------
+
+The procedure to get a native feeling using the AlekSIS PWA varies from
+platform to platform. On some, you are prompted to add AlekSIS to your
+home screen of desktop using a popup; on others, you have to take action
+yourself and find the corresponding menu entry. As of the time of
+writing, “installable” PWAs are supported by all major platforms except
+Firefox Desktop which nevertheless supports the other features.
+
+-  Chromium-based browsers (e.g. Chromium, Google Chrome, Microsoft
+   Edge) will usually prompt you to install the PWA by a popup on both
+   mobile and desktop devices; for the former using a banner and for the
+   latter using an appearing button in the adress bar. In both cases, a
+   click on the notification is enough to start the installation
+   process.
+-  Firefox Mobile will also probably prompt you using a dot near the
+   menu button; then “install” has to been clicked.
+-  On Safari you need to open the share popup and click on the “Add to
+   Home Screen” button.
+
+Independent of the used platform, AlekSIS can be accessed as an
+independent application entry, just like any other installed native
+application, after installation.