Doc: Rearrange best practices and how to documentation

Change the best practices to how-tos, which are categorized
now on a new page.

Fixes: QTBUG-118044
Pick-to: 6.6 6.5
Change-Id: I78dba2a1fde03b346f110ecd54e11485a0869540
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
(cherry picked from commit 7372b3ab6748b68f1c5fba1f61df8083ce719bc5)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/doc/src/custom-types.qdoc

@@ -6,7 +6,7 @@
     \title Creating Custom Qt Types
     \brief How to create and register new types with Qt.
 
-    \ingroup best-practices
+    \ingroup how-to
 
     \tableofcontents
 

src/corelib/doc/src/timers.qdoc

@@ -6,7 +6,7 @@
     \title Timers
     \brief How to use Qt timers in your application.
 
-    \ingroup best-practices
+    \ingroup how-to
 
     QObject, the base class of all Qt objects, provides the basic
     timer support in Qt. With QObject::startTimer(), you start a

src/dbus/doc/src/dbus-adaptors.qdoc

@@ -6,7 +6,7 @@
     \title Using Qt D-Bus Adaptors
     \brief How to create and use DBus adaptors in Qt.
 
-    \ingroup best-practices
+    \ingroup how-to
 
     Adaptors are special classes that are attached to any QObject-derived class
     and provide the interface to the external world using D-Bus. Adaptors are

src/gui/doc/src/richtext.qdoc

@@ -14,7 +14,6 @@
     \ingroup explanations-ui
     \ingroup frameworks-technologies
     \ingroup qt-basic-concepts
-    \ingroup best-practices
 
     \nextpage Rich Text Document Structure