Don't install API man pages

These weren't particularly useful, and the project will be migrating away from
Doxygen entirely anyway.  The serdi man page, which is manually written, of
course remains.

NEWS

@@ -1,6 +1,7 @@
 serd (0.30.7) unstable;
 
   * Deprecate serd_uri_to_path()
+  * Don't install API man pages
   * Fix potential memory error when serialising URIs
   * Move headers to an include directory
   * Refuse to write relative URI references to NTriples
@@ -8,7 +9,7 @@ serd (0.30.7) unstable;
   * Split up and reorganize unit tests
   * Use aligned allocation via C11 or Windows API where possible
 
- -- David Robillard <d@drobilla.net>  Sat, 14 Nov 2020 11:25:26 +0000
+ -- David Robillard <d@drobilla.net>  Thu, 17 Dec 2020 09:09:31 +0000
 
 serd (0.30.6) stable;
 

doc/reference.doxygen.in

@@ -1847,7 +1847,7 @@ RTF_SOURCE_CODE        = NO
 # classes and files.
 # The default value is: NO.
 
-GENERATE_MAN           = YES
+GENERATE_MAN           = NO
 
 # The MAN_OUTPUT tag is used to specify where the man pages will be put. If a
 # relative path is entered the value of OUTPUT_DIRECTORY will be put in front of