Documentation: remove documentation stubs and common-cmds.h generation

Now that we have a manpage in place, we can remove the manpage-style
text files from the Documentation directory.
This allows us also to get rid of the crude common-cmds.h generation,
which relied on these files and on a command-list.txt file.
Instead include the version of that header file generated with the
current HEAD into the source tree.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>