manual/user guide/customization: re-order introduction

This patch re-orders the enumeration of typical project customization
actions:
- move the 'patching packages' section downwards.
- group the entries on post-build scripts and rootfs-overlays to match the
  actual section text.

Since indented enumerations use asterixes, update the top-level entries with
asterixes too for consistency.

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