fix compilation of the doc and started a page dedicated to high performance and or BLAS users
diff --git a/doc/snippets/MatrixBase_extract.cpp b/doc/snippets/MatrixBase_extract.cpp
index 02f44c8..a5ccd75 100644
--- a/doc/snippets/MatrixBase_extract.cpp
+++ b/doc/snippets/MatrixBase_extract.cpp
@@ -1,3 +1,5 @@
+#warning deprecated
+/* deprecated
Matrix3i m = Matrix3i::Random();
cout << "Here is the matrix m:" << endl << m << endl;
cout << "Here is the upper-triangular matrix extracted from m:" << endl
@@ -6,3 +8,4 @@
<< m.part<Eigen::StrictlyUpperTriangular>() << endl;
cout << "Here is the unit-lower-triangular matrix extracted from m:" << endl
<< m.part<Eigen::UnitLowerTriangular>() << endl;
+*/
\ No newline at end of file