manual: file organization: rename files to match new section title

After the renaming of some sections, rename the corresponding files for
clarity.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/manual.txt

@@ -24,9 +24,9 @@ include::prerequisite.txt[]
 
 include::getting.txt[]
 
-include::using.txt[]
+include::quickstart.txt[]
 
-include::get-involved.txt[]
+include::resources.txt[]
 
 = User guide