Unified interface `ncells` for all mesh types

I recognized this while reviewing https://github.com/trixi-framework/Trixi2Vtk.jl/pull/47. Right now, we don't have a unified interface to access the number of cells of a given `mesh`. Based on our [naming conventions](https://trixi-framework.github.io/Trixi.jl/stable/conventions/#Cells-vs.-elements-vs.-nodes) and the existing example of `ncells(mesh::P4estMesh)`, I propose to implement a method `ncells` for each mesh type and document it as official API.

When we do that, we should update Trixi2Vtk accordingly.

CC @andrewwinters5000

Start by locating the existing ncells(mesh::P4estMesh) method and the other mesh types that need corresponding methods. Then inspect the Trixi2Vtk integration mentioned in the issue; done means every mesh type has the documented ncells API and Trixi2Vtk uses the unified interface.

Written by the indexing model from the issue text.