Add a man page.

Signed-off-by: gregory guy <gregory-tde@laposte.net>

doc/man/CMakeLists.txt

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+INSTALL(
+    FILES ${PROJECT_NAME}.1
+    DESTINATION ${MAN_INSTALL_DIR}/man1
+    COMPONENT doc
+)

doc/man/mathemagics.1

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+.TH mathemagics 1 "August 10, 2020"
+.SH NAME
+mathemagics \- a RPN scientific calculator.
+.SH SYNOPSIS
+.B mathemagics
+.RI [Qt-options] [TDE-options] [URL]
+.SH DESCRIPTION
+Mathemagics is a full-featured scientific calculator. It uses Reverse 
+Polish Notation or RPN. mathemagics can store and use user-defined 
+functions and variables, It comes with large number of built-in 
+functions.
+.PP
+.SH OPTIONS
+.SS 
+.SS Generic Options:
+.TP
+.B  --help
+Show help about options
+.TP
+.B  --help-qt                 
+Show Qt specific options
+.TP
+.B  --help-tde                
+Show TDE specific options
+.TP
+.B  --help-all                
+Show all options
+.TP
+.B  --author                  
+Show author information
+.TP
+.B  -v, --version             
+Show version information
+.TP
+.B  --license                 
+Show license information
+.SS
+.PP
+.SH HOMEPAGE
+Homepage: https://mirror.git.trinitydesktop.org/gitea/TDE/mathemagics
+.SH AUTHOR
+This manual page was written by gregory guy <gregory-tde@laposte.net>,
+for the Trinity Desktop Environment project (but may be used by others).