docs: update IV formula syntax

[s3alfisc]

Summary

• document Formulaic's bracketed IV syntax for PyFixest 0.70
• update IV examples in the IV vignette, formula-syntax tutorial, and Skills page
• retain the legacy fixest-style syntax as an explicitly deprecated migration path, linked to the changelog

Why

The formula-parser release makes [endogenous ~ instruments] the supported IV form while the former pipe-delimited form remains temporarily available with a DeprecationWarning.

Validation

• pixi run -e py312-r pytest tests/test_formulaic_compat.py -x -q --no-cov
• rendered the IV, Formula Syntax, and Skills pages with the pinned docs runtime
• fitted all IV formulas shown in the updated examples

[leostimpfle]

Sorry to have missed this in my PR. This looks good, shall I merge?