Casting and documentation fixes (#608)

Add `-wDocumentation` option. Fix the documentation.
Fix c++20/c++17 confusion in tests.

Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

src/ftxui/dom/gridbox.cpp

@@ -33,12 +33,12 @@ int Integrate(std::vector<int>& elements) {
 class GridBox : public Node {
  public:
   explicit GridBox(std::vector<Elements> lines) : lines_(std::move(lines)) {
-    y_size = (int)lines_.size();
+    y_size = lines_.size();
     for (const auto& line : lines_) {
-      x_size = std::max(x_size, (int)line.size());
+      x_size = std::max(x_size, int(line.size()));
     }
     for (auto& line : lines_) {
-      while (line.size() < (size_t)x_size) {
+      while (line.size() < size_t(x_size)) {
         line.push_back(filler());
       }
     }