Maintainer documentation for root_cxx_standard

### Comment:

@vepadulano @chrisburr Can we get a documentation page for `root_cxx_standard` that is similar to what `eigen` has done in https://github.com/conda-forge/eigen-feedstock/blob/main/recipe/README.md for `eigen-abi-devel`? It would be useful for maintainers of other conda-forge recipes that have dependencies on `root_base` to also have some documentation on when and how to depend on `root_cxx_standard`.