docs: fix build error

`docs/macros.itexi`:
Add a newline to ensure expanded macro has `@end ifhtml` or `@end ifnothtml` on their own lines. Related building errors:
````
/home/jimmy/mir/marsyas/doc/contributing.texi:28: misplaced }
/home/jimmy/mir/marsyas/doc/contributing.texi:30: misplaced }
````

`docs/tour.texi`:
Change subsections to sections to supress errors below while building html:
````
/home/jimmy/mir/marsyas/doc/tour.texi:44: raising the section level of @subsection which is too low
````

Tested with texinfo 5.1.