Cabal docs lack description of program-options section

`new-build` appears to now support a `program-options` stanza to specify flags for programs like haddock and preprocessors (for example, see the cabal.project file in this repo). But this stanza is not documented or described anywhere on readthedocs.

Start by inspecting the repository's Read the Docs content and the cabal.project example mentioned in the issue to confirm how the program-options stanza is used. Document the stanza, its purpose for tools such as haddock and preprocessors, and a representative example; it is done when this behavior is described on Read the Docs.

Written by the indexing model from the issue text.