Add Quarto environment variable specification/documentation

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Description

Currently, Quarto has several environment variable such as QUARTO_R and QUARTO_PYTHON, but there is absolutely nothing about those.
The variables are mentioned but never defined:

• https://quarto.org/docs/computations/python.html#installation
• https://quarto.org/docs/computations/r.html#installation
• https://quarto.org/docs/projects/environment.html#overview

Determine which versions of Python and R are used (e.g. QUARTO_PYTHON, QUARTO_R, PY_PYTHON, or RETICULATE_PYTHON).

In the end users need to figure out what those are and what values are allowed on their own.

Another related issue is that a user can set QUARTO_PYTHON=/very/bad/path and still get Quarto to work without any warnings.

> QUARTO_PYTHON="/bad/path" && quarto render index.qmd 

Executing 'index.quarto_ipynb'
  Cell 1/1: ''...Done

pandoc 
  to: html
  output-file: index.html
  standalone: true
  section-divs: true
  html-math-method: mathjax
  wrap: none
  default-image-extension: png
  
metadata
  document-css: false
  link-citations: true
  date-format: long
  lang: en
  title: Quarto Playground
  
Output created: index.html

Start with the Python, R, and project-environment documentation pages linked in the issue, then reproduce the shown render command with an invalid QUARTO_PYTHON value. Done means the supported environment variables and accepted values are documented and the issue’s requested handling of an invalid path is clarified or addressed.