docs: Tagging of examples added

docs/users_guide/examples/hello_units.md

@@ -1,3 +1,9 @@
+---
+tags:
+  - International System
+  - Text formatting
+---
+
 # `hello_units`
 
 This is a really simple example showcasing the features of the **mp-units** library.

docs/users_guide/examples/introduction.md

@@ -0,0 +1,6 @@
+# Introduction
+
+All usage examples in this chapter are categorized with appropriate tags to simplify navigation and search
+of relevant code.
+
+[TAGS]