This adds fairly comprehensive guidance to the solana-program crate docs.

Topics covered:

• The difference between solana-program and solana-sdk
• Where to find examples in the spl
• How solana programs differ from most rust programs
• How to conventionally define a solana program
• On-chain vs off-chain targets
• Core data type descriptions
• Serialization strategies
• CPI
• Native programs, distinction between different kinds, where to find them
• Sysvars
• Limitations to the Solana version of Rust std

I also added some docs to the log and sysvar modules.

After this I intend to work top-down making sure all the links in the crate docs lead to useful information, modules all have short descriptions, etc.