DOP-5189: Create Netlify function to proxy URL fetch

[rayangler]

Stories/Links:

DOP-5189

Current Behavior:

N/A - Function does not exist yet

Staging Links:

https://deploy-preview-1310--docs-frontend-stg.netlify.app/.netlify/functions/fetch-url?url=https://raw.githubusercontent.com/10gen/docs-shared/main/dbx/encrypt-fields.rst

https://deploy-preview-1310--docs-frontend-stg.netlify.app/.netlify/functions/fetch-url?url=https://raw.githubusercontent.com/10gen/docs-shared/main/dbx/compatibility-table-legend.rst

https://deploy-preview-1310--docs-frontend-stg.netlify.app/.netlify/functions/fetch-url?url=https://raw.githubusercontent.com/10gen/docs-shared/main/dbx/lifecycle-schedule-callout.rst

Notes:

• The function currently relies on the url query param to fetch the content from said URLs. The function is currently as minimal as possible and leverages basic cache headers.
• This function is intended to only be a temporary solution for 403 errors from sharedinclude files in both Netlify builders and the Autobuilder. This function will ideally be removed once we no longer need to support sharedinclude after the monorepo is all set up (will file a new ticket for this once this ticket is complete).
• This function will be used by the parser to conditionally get content from raw.githubusercontent.com URLs. See the parser PR for more information.
• Netlify builders examples: With 403 errors vs. No 403 errors
• Autobuilder examples: With 403 errors vs. No 403 errors

README updates

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

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