Add NEWS bullet

Author: cderv

NEWS.md

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 # quarto (development version)
 
+- Added `project_path()`, `get_running_project_root()`, and `find_project_root()` functions for Quarto-aware project path construction. These functions provide a consistent way to reference files relative to the project root, working both during Quarto rendering (using `QUARTO_PROJECT_ROOT` environment variables) and in interactive sessions (using intelligent project detection). The `project_path()` function is particularly useful in Quarto document cells where you need to reference data files or scripts from the project root regardless of the document's location in subdirectories (#180).
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 - `quarto_preview()` now explicitly returns the preview server URL (invisibly) and documents this behavior. This enables programmatic workflows such as taking screenshots with **webshot2** or passing the URL to other automation tools (thanks, @cwickham, #233).
 
 - Added NA value detection in YAML processing to prevent silent failures when passing R's `NA` values to Quarto CLI. Functions `as_yaml()` and `write_yaml()` now validate for NA values and provide clear error messages with actionable suggestions. This addresses issues where R's `NA` values get converted to YAML strings (like `.na.real`) that Quarto doesn't recognize as missing values, because they are not supported in YAML 1.2 spec. This is to help users handle missing data appropriately before passing to Quarto (#168).