Docs Reorganization and Rewrite

[s3alfisc]

We want to reorganize the documentation from a flat structure into a Diataxis-inspired layout with two main content sections:

• Tutorials — Learning-oriented walkthroughs for new users (OLS, IV, GLMs, standard errors, DiD, etc.)
• Guides — Practical recipes and deeper dives for practitioners (marginal effects, multiple testing, GPU, Gelbach decomposition, small sample corrections, etc.)
• Textbook Replications — Reproductions of examples from causal inference textbooks

What changed

What has already happened in #1173:

• Reorganize docs into tutorials and guides
• Break monolith quickstart into multiple tutorials
• Use grid for tutorials and guides index pages

To do

• Rewrite fixed effect tutorial
• Write IV tutorial
• Write GLM tutorial
• Write inference tutorial
• Clean up getting started page
• Write multiple estimation page
• Add variance reduction notebook to guides
• Add notebook on gender wage gap composition with PyFixest to comply with EU Pay Gap Directive
• Statistical Documentation: overview of core formulas and estimation approaches
• How to guides: Surrogate indices, experimental debiasing (Coey Cunningham), other?

Relates to #541

[leostimpfle]

I'd be interested in contributing to the Poisson part of the GLM tutorial. I think it would be great to go beyond count data and reference "modern" applications of PQML following Wooldridge (2024) and Chen and Roth (2024).