Only export public declarations into the website

[tothambrus11]

Currently, there is no filtering for symbols in the website generation. Readers are only interested in declarations that they can use from outside the module, so we should filter for public declarations. Refer to https://github.com/hylo-lang/specification/blob/main/spec.md#modifiers for details.

Relevant files from Hylo frontend: AccessModifier.swift, all files within Frontend/AST/Decl.
Relevant folder from HyloDoc: Sources/WebsiteGen/

[tothambrus11]

It is currently a bit convoluted the way we render members in traits and product type pages: we convert them to targets, filter them and convert them back. This should be also simplified.
isSupportedDecl in PartialTargetResolution.swift should be removed since we support everything except stuff where we don't traverse into anymore. It should be replaced with another function that checks if the declaration is publicly exposed.