UPD: Improve documentation for version 0.9.2

[tasseff]

This pull request improves the documentation for version 0.9.2. This documentation update provides:

• A thorough description of the network data format;
• A thorough description of the result data format;
• A mathematical description of the foundational steady-state model used by WaterModels;
• High-level descriptions of the various network formulations provided by WaterModels;
• Descriptions of the problem specifications supported by WaterModels;
• Complete documentation of all objective, variable, and constraint creation functions;
• Developer documentation regarding naming conventions; and
• A small description of the examples provided within the examples directory.