maint: rename rules to build/upload web manuals

maint: rename rules to build/upload web manuals

The new names are the same used by the similar-purpose rules provided by
the 'top/maint.mk' makefile in gnulib (as of commit v0.0-7741-g4a8c422).
No need to have gratuitous incompatibilities.

* Makefile.am (web-manuals): Rename ...
(web-manual): ... like this.
(upload-web-manuals): Rename ...
(web-manual-update): ... like this.
(clean-web-manuals): Rename ...
(clean-web-manual): ... like this.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>