lenovo/yoga/7/pro-x: add 14IAH7; create shared for 14ARH7 and 14IAH7

[sebaszv]

Description of changes

nixos-hardware already had Lenovo 14ARH7 (Lenovo Yoga Slim 7 Pro X) model
profiles. This an AMD device with an NVIDIA dedicated GPU. There is a profile
for integrated graphics only and another for hybrid graphics.

There is an Intel version of the same device, the Lenovo 14IAH7 (Lenovo Yoga
Slim 7i Pro X). Seeing as they are primarily the same device, save the
difference in processor under the Lenovo Yoga Slim 7 Pro X "line", the listed
was done to share values between the AMD version and the Intel version that was
added in this commit. It follows the same pattern of having a profile for
integrated graphics only and another for hybrid graphics.

The implementation for the 14ARH7 modules was changed during this, but the goals
were entirely maintained. There was a considerable amount of redundant rewriting
of values that were in imported modules. An effort to remove unnecessary
declarations in favour of importing common modules was done. "Optional" values
were assigned with lib.mkDefault as well, such as for
hardware.nvidia.nvidiaSettings. According to CONTRIBUTING.md, this is
encouraged.

The 14ARH7 nixosModule names were left as is to not cause unnecessary
breakages.

• Create 'pro-x' subsection in lenovo/yoga/7
• Pull content from '14ARH7' profiles to share with '14IAH7' in
  lenovo/yoga/7/pro-x/shared
• Strip '14ARH7' profiles to use shared modules and only have unique values
• Strip unnecessary declarations in favour of importing common modules
• Create profiles for '14IAH7'
• Update flake.nix and README.md

Things done

• Tested the changes in your own NixOS Configuration
• Tested the changes end-to-end by using your fork of nixos-hardware and
  importing it via <nixos-hardware> or Flake input

[sebaszv]

Should the deprecated profile lenovo-yoga-7-14ARH7 be removed now as well with this restructuring?

The commit that deprecated profile in master is 5 months old at this point:
5bd0371