Hmm, well the last time I did this was to get information for designing Intel Atom into an embedded system- this was ~2019 (I work for a small design firm). I remember you had to sign up for a premier account, sign a CNDA, then ask them to add the specific chipset documentation to your account. Maybe they were forgiving because these were low-end embedded chips.

Anyway, the information for the PCIe registers is in documents like this:

https://theretroweb.com/misc/documentation/324642-003-67f43c...