5.3.1.2 Construction of sections

The base of the pyramid \( S A B C D \) is a rhombus with diagonals \( A C=a, B D=b \). The side edge \( S A \) is perpendicular to the plane of the base and is equal to c. Construct a section of the pyramid with a plane passing through the point \( A \) and the midpoint \( K \) of the edge \( S C \) parallel to the straight line \( B D \), and find the area of ​​this section.