Add 2% Carbopol-in-glycerin HB/TC documentation walkthrough

[Copilot]

This adds a new case-study walkthrough for a 2% Carbopol in glycerin flow curve, focused on comparing Herschel–Bulkley and TC fits and on interpreting the effect of a viscous continuous phase. It also archives the sample TRIOS JSON used by the walkthrough so the example is reproducible from the docs.

• New walkthrough

  • Added docs/walkthrough-carbopol-glycerin.md
  • Covers:
    • loading and step discovery for the sample JSON
    • step-by-step HB fitting
    • step-by-step TC fitting
    • side-by-side interpretation of fit outputs
    • discussion of how glycerin shifts the high-shear response into the continuous-phase viscosity term

• Bundled documentation sample

  • Added docs/walkthrough/cp02_gly_20c_fc.json
  • Keeps the walkthrough self-contained and reproducible from the repository

• Visual comparison assets

  • Added fit-comparison and TC-decomposition figures:
    • docs/walkthrough/fig11_cp02_gly_tc_vs_hb.png
    • docs/walkthrough/fig12_cp02_gly_tc_decomposition.png
  • These support the walkthrough’s comparison of empirical fit quality vs physical interpretability

• Docs navigation

  • Linked the new case study from docs/index.md

Example from the new walkthrough:

import rheofit

rheofit.print_steps("walkthrough/cp02_gly_20c_fc.json")
df = rheofit.load_step("walkthrough/cp02_gly_20c_fc.json", 0)

hb = rheofit.fit(df, "herschel_bulkley", effort="thorough", seed=0)
tc = rheofit.fit(df, "tc", effort="thorough", seed=0)

• Fixes Add documentation walkthrough: HB and TC fit for 2% Carbopol in Glycerin sample #27