manual: file organization: avoid unnecessary indirection

Some source files of the manual merely contain inclusion of other files.
Especially at top-level this is unnecessary, and one could just as well add
these includes in manual.txt.

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