Add hydrogen benchmark comparison

[ohno]

Summary

• add a hydrogen-atom benchmark table comparing the two lowest s-wave energies
• include RR, FC, FDM, QTT+DMRG, VNN, VMC, and exact reference values
• document the numerical settings used for each method and mark unavailable excited states

QTT is already available on main via #38, so the table reports its DMRG solver as QTT+DMRG.

Validation

• QTT tests: 82/82 passed
• full Documenter build passed with DOCUMENTER_LOCAL=true julia --project=docs docs/make.jl

Close #37

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This pull request was developed with assistance from Codex using GPT-5.6 Sol with High reasoning effort.