Generate documentation from build directory

When man pages should be included they need to be copied from man/*.rst.
This means documentation can no longer be built from the static reST
files alone (which are referenced using “abs_top_srcdir”). Similar to
Python files, automake doesn't copy or link the input files for
documentation into the build tree.

This patch adds all files required to build the documentation to
“srclink_files”, renames the “docrst" variable to “docinput”, and then
references the files in the build tree using “abs_top_builddir”.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>