Document eleventyConfig.directories (shipped with v3.0.0)

`eleventyConfig.directories`

https://github.com/11ty/eleventy/issues/2729#issuecomment-2043095000
https://github.com/11ty/eleventy/pull/3244

Notable cross-reference to `eleventy.directories` in Eleventy Supplied Data: https://www.11ty.dev/docs/data-eleventy-supplied/#eleventy-variable