[web] Improvement of the document of the Theis' problem

This MR improves the document of the Theis' problem by

• adding the governing equation,
• adding parameters and their description,
• adding references regarding the parameters and the numerical solution,
• more result figures, consistent with that in the references,
• adding a conclusion section,
• adding a schematic image.

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behaviour change was documented?