Create cmd to generate man pages for prof commands

`profanity mangen` will create for each command (`/account`, `/roster`)
an own manpage (`profanity-account.1`, `profanity-roster.1`)

See https://github.com/profanity-im/profanity/issues/1444

Needs some polishing formatting wise.

src/command/cmd_defs.h

@@ -50,6 +50,7 @@ GList* cmd_get_ordered(const char* const tag);
 gboolean cmd_valid_tag(const char* const str);
 
 void command_docgen(void);
+void command_mangen(void);
 
 GList* cmd_search_index_all(char* term);
 GList* cmd_search_index_any(char* term);