Add examples to docs

docs/make.jl

@@ -11,6 +11,7 @@ makedocs(;
     ),
     pages = [
         "Home" => "index.md",
+        "Examples" => "examples.md",
         "API" => "api.md"
     ],
     repo = "https://git.km3net.de/common/KM3io.jl/blob/{commit}{path}#L{line}",

docs/src/examples.md

 # Examples
 
-## Whatever
+## Offline data
 
-```@example
+Let's use the `KM3NeTTestData` Julia package which contains all kinds of KM3NeT
+related sample files. The `datapath()` function can be used to get a path to
+such a file. In the following, we will discover the `numucc.root` file which
+contains 10 muon neutrino charged current interaction events.
+
+```@example 1
 using KM3io, KM3NeTTestData
 
 f = ROOTFile(datapath("offline", "numucc.root"))
 ```
 
+The `ROOTFile` is the container object which gives access to both the online and
+offline tree. In this case, the online tree is empty
+
+```@example 1
+print(f.online)
+```
+
+and the offline tree holds our 10 MC events:
+
+```@example 1
+print(f.offline)
+```