remove deprecated fields/functions, clean up docs

Mostly documentation cleanups, but this commit does remove a number of
deprecated fields and functions:

* `ghw.PCIInfo.Classes` (deprecated for >1 year, use pcidb instead)
* `ghw.PCIInfo.Products` (deprecated for >1 year, use pcidb instead)
* `ghw.PCIInfo.Vendors` (deprecated for >1 year, use pcidb instead)
* `ghw.PCIInfo.ListDevices()` (deprecated for >1 year, use
  `ghw.PCIInfo.Devices`)
* `ghw.pkg/pci.Address` (deprecated for >1 year, just use
  pkg/pci/address package)
* `ghw.pkg/pci.AddressFromString()` (deprecated for >1 year, just use
  pkg/pci/address package)

Primary documentation changes were to move the configuration options,
environment-setting instructions, serialization and snapshot stuff into
an "Advanced Usage" section towards the bottom of the README and focus
the main "Usage" part on the domain-specific functions like `CPU()`,
`Memory()` etc.

Signed-off-by: Jay Pipes <jaypipes@gmail.com>