Move doxygen snippets to separate file

UltiSnips/c_doxygen.snippets

@@ -0,0 +1,19 @@
+snippet "/// p" "Doxygen param tag"
+/// @param${1/.+/[/}$1${1/.+/]/} $0
+endsnippet
+
+snippet "/// b" "Doxygen brief tag"
+/// @brief $0
+endsnippet
+
+snippet "/// r" "Doxygen return tag"
+/// @return $0
+endsnippet
+
+snippet "/// n" "Doxygen note tag"
+/// @note $0
+endsnippet
+
+snippet copydoc "Doxygen copydoc block"
+/// @copydoc $0
+endsnippet