Documentation update

[ianthomas23]

Updates to documentation:

• Split the top-level API docs into 4 pages: enums, contour_generator, ContourGenerator, other.
• Add pages on manipulating contoured data (convert_* and dechunk_* functions).
• Fix Renderer-derived __init__ docstrings.
• Add new section on using with external libraries starting with Shapely.
• Clarify that can use strings instead of enums in some places.
• Clarify use of -np.inf and np.inf for filled contours.