docs: improve accuracy for self-hosted OpenAPI spec (#10318)
### Description
The self-hosted OpenAPI spec is generated from the Vercel Remote Cache
spec. However, that there can be spots where the Vercel Remote cache
spec has information that the self-hosted spec doesn't need.
This PR introduces some light pre-processing so that we can make the
self-hosted spec better for OSS-only users. See
https://github.com/vercel/turborepo/pull/10318/files#diff-03cadc255ad9d43cf46b7ed6185da0eba06ef11294de0f9a784274a1863b6aa0R1
for more.
### Testing Instructions
👀
https://turbo-site-git-shew-240ef.vercel.sh/api/remote-cache-spec now
doesn't have `examples` fields, since the Vercel Remote Cache spec has
examples specific to Vercel.
This also removes the erroneous example string from
https://turbo-site-git-shew-240ef.vercel.sh/docs/openapi/artifacts/download-artifact.
