Port `pci` device documentation to `gendoc`

[stgraber]

The PCI device documentation (doc/reference/devices_pci.md) currently uses a static table of valid options. This should be moved to an auto-generated table using gendoc.

This is typically done in two steps:

1. Add the required gendoc comments where the configuration options are defined (usually alongside their validator)
2. Generate the new config metadata (make update-metadata)
3. Update the documentation to use the generated metadata

See https://github.com/lxc/incus/pull/847 for an example.

[DinglyCoder]

Hello, @pgupta2005 and I are University students at the University of Texas at Austin, we would like to take on this issue as part of our Virtualization class with Professor Vijay Chidambaram

[parthgupta2005]

Hello, we are excited to resolve the issue!

[stgraber]

Assigned the two of you, thanks!