Update sunbird_portal_user_upload_ref_link

[ArchanaKumari01]

Document added for sunbird_portal_user_upload_ref_link portal environment variables

• [ ] Have you followed Contribution Guidelines

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[lakhanmandloi]

Hello @ArchanaKumari01,

This is a auto-generated response.
Click the URL for review pull request - http://sunbird-docs-qa.s3-website.ap-south-1.amazonaws.com/pr/673.

Modified pages are -
- http://sunbird-docs-qa.s3-website.ap-south-1.amazonaws.com/pr/673/developer-docs/configuring_sunbird/env_variables_portal

Modified sections are -

[Sunitashelar]

This is incorporated in the latest published content in the portal services environment variables page. hence , closing this PR without merging it.