examples: add project demonstrating a junction element

This is part of the work towards #437.

doc/examples/junctions/elements/callHello.bst

+kind: import
+
+sources:
+- kind: local
+  path: files/callHello.sh
+
+depends:
+  - filename: hello.bst
+    junction: hello-junction.bst

doc/examples/junctions/elements/hello-junction.bst

+kind: junction
+
+# Specify the source of the BuildStream project
+# We are going to use the autotools examples distributed with BuildStream in the
+# doc/examples/autotools directory
+sources:
+- kind: local
+  path: ../autotools

doc/examples/junctions/files/callHello.sh

+#!/bin/sh
+echo "Calling hello:"
+hello

doc/examples/junctions/project.conf

+# Unique project name
+name: junctions
+
+# Required BuildStream format version
+format-version: 9
+
+# Subdirectory where elements are stored
+element-path: elements