Fix internal links in docs/ (#2080)

One link in the summary (Modding your keyboard) referred to a page that
was deleted; the rest have been updated to point to the new names.

docs/understanding_qmk.md

@@ -2,7 +2,7 @@
 
 This document attempts to explain how the QMK firmware works from a very high level. It assumes you understand basic programming concepts but does not (except where needed to demonstrate) assume familiarity with C. It assumes that you have a basic understanding of the following documents:
 
-* [QMK Overview](qmk_overview.md)
+* [Introduction](getting_started_introduction.md)
 * [How Keyboards Work](how_keyboards_work.md)
 * [FAQ](faq.md)