Fix cross-reference slugs

[cncastillo]

Documenter 1.16 tightened its cross-reference checks, so links like [ADC](@ref) and [Phantom](@ref) started failing because multiple headings share the same slug. This patch points those references at their concrete definitions (KomaMRIBase.ADC, KomaMRIBase.Sequence, KomaMRIBase.Phantom), giving Documenter unique targets again and unblocking the docs build.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.14%. Comparing base (9351546) to head (cf1ee75).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #646      +/-   ##
==========================================
+ Coverage   90.00%   90.14%   +0.13%     
==========================================
  Files          57       57              
  Lines        3233     3228       -5     
==========================================
  Hits         2910     2910              
+ Misses        323      318       -5     

Flag	Coverage Δ
base	89.08% <ø> (ø)
core	89.73% <ø> (+0.78%)	⬆️
files	94.05% <ø> (ø)
komamri	88.21% <ø> (ø)
plots	90.90% <ø> (ø)

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see 1 file with indirect coverage changes

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