Do not document methods on enum types

[greschd]

• Rename the 'Other data types' API section to 'Enumeration data types'
• Create an autosummary template which skips the methods section, and use it for the enum data types
• Rename "Elemental and nodal data types" to "Mesh data types"
• Move non-enum classes MeshData and TriangleMesh to "Mesh data types"
• Rename the custom autosummary templates by adding the .jinja2 suffix, to make the vale style check ignore them.