Extract documentation for named constants

Since feature flags are used by rules to enable behaviors (sanitizers, etc.), and they can be used globals with a bazel command line or via the common `features` rule attribute.

Generally, the features are just string values. Some projects use a constant in their bzl so they can ensure the same value is used in all cases the impls need it. That pattern generally works to provide a basis extracting the information into documentation. _rules_swift_ is an example of this: https://github.com/bazelbuild/rules_swift/blob/master/swift/internal/features.bzl#L17-L115

So it would be nice if there was some way to scrap the string values and descriptions to make a page.

Start by reviewing the referenced rules_swift/internal/features.bzl example and how its named constants pair string values with descriptions. Stardoc files and tests are not named, so locate the existing documentation-generation entry points before defining the extraction scope. Done should produce a documentation page containing the feature values and descriptions.

Written by the indexing model from the issue text.