Rename Introduction to User Guide and move Tutorials next to it (#701)

`User Guide` is a more suitable name for the section that contains the
basic information about the SDK and how to use it. It is also more
broad, as it allow us to also include more advanced topics in the same
section.

Having the `Tutorials` section next to it seems like a more natural
order for reading the documentation. The users can read the user guide
first and then go through the tutorials to have a more hands-on
experience.

Author: llucax

docs/SUMMARY.md

@@ -1,5 +1,5 @@
 * [Home](index.md)
-* [Introduction](intro/)
-* [API Reference](reference/)
+* [User Guide](user-guide/)
 * [Tutorials](tutorials/)
+* [API Reference](reference/)
 * [Contributing](CONTRIBUTING.md)

docs/_scripts/macros.py

@@ -55,7 +55,7 @@ def glossary(term: str) -> str:
             The Markdown link to the glossary entry for the given term.
         """
         current_path = pathlib.Path(env.page.file.src_uri)
-        glossary_path = pathlib.Path("intro/glossary.md")
+        glossary_path = pathlib.Path("user-guide/glossary.md")
         link_path = glossary_path.relative_to(current_path.parent)
         return f"[{term}]({link_path}#{_slugify(term)})"