sidekick: refactor goldmark to internal/markdown package

we use goldmark in Rust, and will probably need it for other languages too. it might make sense to make our own internal/markdown package similar to internal/yaml so all the options are the same across the repo

_Originally posted by @julieqiu in https://github.com/googleapis/librarian/pull/5886#discussion_r3201547564_

Start by reviewing the repository's current goldmark usage and the existing internal/yaml package. Determine the shared options that an internal/markdown package should provide across the repo, including the Rust-related use mentioned in the issue. Done means goldmark usage is refactored through internal/markdown with consistent options.

Written by the indexing model from the issue text.