doc: add snake_case section for C-like structs

This commit adds a section mentioning that for C-like structs it is
alright to use snake_case.

PR-URL: #20423
Reviewed-By: Anna Henningsen <[email protected]>

Author: danbev

CPP_STYLE_GUIDE.md

@@ -12,6 +12,7 @@
   * [CamelCase for methods, functions, and classes](#camelcase-for-methods-functions-and-classes)
   * [snake\_case for local variables and parameters](#snake_case-for-local-variables-and-parameters)
   * [snake\_case\_ for private class fields](#snake_case_-for-private-class-fields)
+  * [snake\_case\_ for C-like structs](#snake_case_-for-c-like-structs)
   * [Space after `template`](#space-after-template)
 * [Memory Management](#memory-management)
   * [Memory allocation](#memory-allocation)
@@ -147,6 +148,15 @@ class Foo {
 };
 ```
 
+## snake\_case\_ for C-like structs
+For plain C-like structs snake_case can be used.
+
+```c++
+struct foo_bar {
+  int name;
+}
+```
+
 ## Space after `template`
 
 ```c++