manual/user guide/customization: rework section on rootfs customization

This patch reworks the section on root filesystem customization as follows:
- use labeled list instead of bulleted list to clarify the different methods
- move rootfs overlay and post-build scripts to the top and label them as
  recommended.
- split post-image to a separate section, as it is not related to the target
  filesystem customization
- line up post-build and post-image explanations, for example regarding
  working directory of the script
- general expansion of some of the explanation
- general rewording

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>