[Docs] Fix sizes of PWA screenshots

docs/user/20_pwa.rst

@@ -37,7 +37,7 @@ Edge) will usually prompt you to install the PWA by a popup on both
 mobile and desktop devices; for the former using a banner
 
 .. image:: ../_static/pwa_mobile_chromium.png
-  :width: 100%
+  :width: 40%
   :alt: PWA installation prompt on the mobile version of the Chromium browser
 
 and for the latter using an appearing button in the address bar.
@@ -53,14 +53,14 @@ Firefox Mobile will also prompt you using a dot near the
 menu button; then ``Install`` has to be clicked.
 
 .. image:: ../_static/pwa_mobile_firefox.png
-  :width: 100%
+  :width: 40%
   :alt: PWA installation prompt on the mobile version of the Firefox browser
 
 On Safari Mobile, you need to open the share popup and click on the
 ``Add to Home Screen`` button.
 
 .. image:: ../_static/pwa_mobile_safari.png
-  :width: 100%
+  :width: 40%
   :alt: PWA installation prompt on the mobile version of the Safari browser
 
 No matter what platform is used, AlekSIS can be accessed as an