Add updated maths and fitting tutorials to the documentation

[markbujehein]

Summary

This PR adds a new, learner-friendly tutorial series to the EasyScience documentation, with a clearer path for new users:

• Basics: working with numbers that carry units, uncertainties, and bounds
• Maths functions: arithmetic, error propagation, trig and inverse trig, exp/log, hyperbolic functions, and correlation
• Fitting: fitting workflows and derived quantities from fitted parameters
• Advanced topics: Bayesian analysis and posterior-based workflows

It also updates the docs landing pages and introduction to direct users into the new tutorial structure.

Why

The current docs were missing a structured onboarding path for users who want to learn EasyScience, so while adding tutorial for the maths module, I'm trying out something that seems more user friendly. These notebooks provide:

• clear examples of EasyScience applied on various problems
• inspired by the DMSC 2026 summer school notebooks, exercises with hints & solutions to reinforce learning
• worked explanations of uncertainty propagation and parameter behaviour
• a natural progression from basics to fitting and Bayesian analysis

What changed

• Added a new Basics tutorial section:
  • arithmetic
  • error propagation (with edge cases)
  • trigonometric functions
  • inverse trigonometric functions
  • exponential/logarithm functions
  • hyperbolic functions
  • correlation (with edge cases)
• Added a Fitting tutorial section:
  • fitting introduction
  • SANS sphere example (ported from Workshops & schools)
  • QENS Lorentzian example (ported from Workshops & schools)
• Added Advanced Topics:
  • Bayesian analysis
  • Bayesian QENS
• Updated the docs homepage and introduction page to point new users toward:
  • Installation & Setup
  • Basics tutorials
  • Fitting tutorials
  • Workshops & schools

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.07%. Comparing base (db1f696) to head (fb4dc5e).

Additional details and impacted files

@@                 Coverage Diff                  @@
##           add-maths-module     #331      +/-   ##
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+ Coverage             84.05%   84.07%   +0.01%     
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  Files                    64       64              
  Lines                  5595     5595              
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+ Hits                   4703     4704       +1     
+ Misses                  892      891       -1     

Flag	Coverage Δ
unittests	84.07% <ø> (+0.01%)	⬆️

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see 1 file with indirect coverage changes

[rozyczko]

1. Putting tutorials in subdirectories broke the Google Colab button
   The template builds the Colab URL from page.nb_url.split('/')[-2] and [-1], i.e. it assumes tutorials/<name>/<name>.ipynb. With use_directory_urls the new notebooks are published at tutorials/basics/<name>/<name>.ipynb, so the generated link drops the basics/ or fitting/ segment.
   Suggested fix (not tested): build the path from the page URL instead of from the last two path segments, e.g.

{% set filename = page.nb_url.split('/')[-1] %}
{% set colab_url =
  base_colab_url ~ config.extra.vars.github_repository ~
  "/blob/gh-pages/" ~ config.extra.vars.docs_version ~
  "/" ~ page.url ~ filename
%}

2. notebook-strip does not see the sub-folders
   In pixi.toml, task notebook-strip = 'nbstripout docs/docs/tutorials/*.ipynb'.
   The notebook-prepare step that both docs.yml and tutorial-tests.yml run before
   executing notebooks only strips the top-level folder. notebook-exec, notebook-tests and nbqa ruff all take the directory and are recursive, so nothing currently breaks. But the first time someone commits a nested notebook with outputs, the docs build will render stale outputs instead of new ones. Make the glob recursive (e.g. nbstripout $(git ls-files 'docs/docs/tutorials/**/*.ipynb') or a
   find … -exec), and consider the same for the commented-out notebook-convert/-tweak.