Add hadron spectroscopy examples

[ohno]

Closes #39.

Summary

• add the requested $\Lambda_c(1/2^+)$ and two $\eta_c(1S)$ spectroscopy examples
• use the Hamiltonian parameters and geometric Gaussian basis sets specified in the issue
• verify the calculated ground-state masses against the published values

Validation

• 554/554 package tests passed
• documentation build and doctests passed
• rendered masses: 2286.000, 3005.252, and 3020.148 MeV

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

[MartinMikkelsen]

Also done here

[ohno]

@MartinMikkelsen
I added more and better examples
#39 (comment)

I will also have this pull request fixed.