Snippets update (#6204)

~~Added explanatory comment about math package usage~~
Changed Main() entry point to Run() as per design and toolchain

This small update of front page code snippets will add
explanatory comment to highlight that provided Carbon code
is  hypothetical and meant to show the look and feel of the language.

Also it delivers change of Main() to Run() to
highlight correct entry point for Carbon lang.

README.md

@@ -169,6 +169,9 @@ If you're already a C++ developer, Carbon should have a gentle learning curve.
 It is built out of a consistent set of language constructs that should feel
 familiar and be easy to read and understand.
 
+The Carbon code here is hypothetical and meant to show the look and feel of the
+language.
+
 C++ code like this:
 
 <a href="docs/images/snippets.md#c">

docs/design/expressions/literals.md

@@ -226,7 +226,7 @@ fn Sum(x: i32, y: i32) -> i32 {
   return x + y;
 }
 
-fn Main() -> i32 {
+fn Run() -> i32 {
   return Sum(4, 2);
 }
 ```

docs/images/snippets.md

@@ -98,7 +98,7 @@ fn PrintTotalArea(circles: [Circle]) {
   Print("Total area: {0}", area);
 }
 
-fn Main() -> i32 {
+fn Run() -> i32 {
   // A dynamically sized array, like `std::vector`.
   var circles: array [Circle] = ({.r = 1.0}, {.r = 2.0});
   // Implicitly constructs a slice from the array.