cdecl: add page (#14285)

Co-authored-by: Wiktor Perskawiec <git@spageektti.cc>

pages/common/cdecl.md

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+# cdecl
+
+> Compose and decode C and C++ type declarations.
+> More information: <https://github.com/paul-j-lucas/cdecl>.
+
+- Compose English phrase into C declaration, and create [c]ompilable output (include `;` and `{}`):
+
+`cdecl -c {{phrase}}`
+
+- Explain C declaration in English:
+
+`cdecl explain {{C_declaration}}`
+
+- Cast a variable to another type:
+
+`cdecl cast {{variable_name}} to {{type}}`
+
+- Run in [i]nteractive mode:
+
+`cdecl -i`