add rewritten cstyleCast.md document

Author: danmar

man/checkers/cstyleCast.md

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+
+# cstyleCast
+
+**Message**: C-style pointer casting<br/>
+**Category**: Modernization<br/>
+**Severity**: Style<br/>
+**Language**: C++, not applicable for C code
+
+## Description
+
+Many developers feel that it's best practice to replace C casts with C++ casts.
+
+This checker is about C casts that converts to/from a pointer or reference.
+
+Potential safety issues are covered by other warnings so this ID `cstyleCast` is primarily about writing warnings for casts that are safe.
+
+## How to fix
+
+You can use C++ casts such as `static_cast` to fix these warnings.
+
+Before:
+```cpp
+struct Base{};
+struct Derived: public Base {};
+void foo(Base* base) {
+    Base *p = (Base*)derived; // <- cstyleCast, safe cast from derived object to base object
+}
+```
+
+After:
+```cpp
+struct Base{};
+struct Derived: public Base {};
+void foo(Base* base) {
+    Derived *p = static_cast<Derived*>(base);
+}
+```