Add documentation for warning C26457 (#3421)

* add documentation for C26457

* update warning page and add entry to table of contents.

* Update c26457.md

* Delete unneeded metadata

Co-authored-by: Colin Robertson <[email protected]>

Author: JordanMaples

docs/code-quality/c26457.md

@@ -0,0 +1,22 @@
+---
+description: "Learn more about: C26457 USE_STD_IGNORE_INSTEAD_OF_VOID_CAST"
+title: C26457
+ms.date: 3/1/2021
+f1_keywords: ["C26457"]
+helpviewer_keywords: ["C26457"]
+---
+# C26457 USE_STD_IGNORE_INSTEAD_OF_VOID_CAST
+
+Excerpt from the [C++ Core Guideline for this warning](https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#es48-avoid-casts):
+
+> Never cast to `(void)` to ignore a `[[nodiscard]]` return value. If you deliberately want to discard such a result, first think hard about whether that is really a good idea (there is usually a good reason the author of the function or of the return type used `[[nodiscard]]` in the first place). If you still think it's appropriate and your code reviewer agrees, use `std::ignore =` to turn off the warning which is simple, portable, and easy to grep.
+
+```C++
+struct S{};
+[[nodiscard]] S getS();
+
+void function() {
+    (void) getS(); // C26457
+    std::ignore = getS(); // OK
+}
+```

docs/code-quality/toc.yml

@@ -137,6 +137,8 @@
     href: ../code-quality/c26455.md
   - name: C26456
     href: ../code-quality/c26456.md
+  - name: C26457
+    href: ../code-quality/c26457.md
   - name: C26460
     href: ../code-quality/c26460.md
   - name: C26461