DOP-5189: Proxy requests to raw GH content

[rayangler]

Ticket

DOP-5189

Notes

• While we continue to support the sharedinclude directive, the parser should resolve URL requests to raw.githubusercontent.com by leveraging a proxy serverless Netlify function. The original URL will be passed in as a query param for convenience. The function will be in charge of returning content to the parser.
• This should be a temporary solution for now and will be removed once the Netlify function is successfully decommissioned, as mentioned in the frontend PR (ticket pending).

README updates

• • This PR introduces changes that should be reflected in the README.md and/or HACKING.md, and I have made those updates.
• • This PR does not introduce changes that should be reflected in the README.md and/or HACKING.md