Provide example input for each example on the website

[durka]

Most of the examples at serde.rs have example input in JSON or YAML, but a few do not. IMO, they all should, so there's no mystery as to what each example is for. To be honest, I've read the "Deserialize for custom map type" over and over and I have absolutely no idea what it does.

• [x] Structs and enums in JSON
• [x] Enum representations
• [x] Default value for a field
• [x] Struct flattening
• [x] Handwritten generic type bounds
• [ ] Deserialize for custom map type
• [x] Array of values without buffering
• [x] Serialize enum as number
• [x] Serialize fields as camelCase
• [x] Skip serializing field
• [ ] Derive for remote crate
• [ ] Manually deserialize struct
• [ ] Discarding data
• [x] Transcode into another format
• [x] Either string or struct
• [x] Convert error types
• [x] Custom date format