docs(t400,macpro31): add per-machine install-note READMEs

Add system/machine/{T400,MacPro31}/README.md covering the placeholder
hardware-configuration regeneration, partition labels, bootloader selection
(T400 boot variants; Mac Pro EFI quirks), and GPU notes. Link each from its
configuration.nix header, and refresh the top-level README host table (T400
replaces X1, Mac Pro 3,1 added) with links to both.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

system/machine/T400/configuration.nix

@@ -1,5 +1,6 @@
 # ThinkPad T400 (NixOS). Shared laptop options live in ../../modules/laptop.nix;
-# only host-specific settings are here.
+# only host-specific settings are here. Install notes (boot variants, GPU,
+# partitions): see ./README.md.
 { ... }:
 
 {