docs: mention using docs.openverse.org links for plans (#5185)

Co-authored-by: Sadiyah Khanam <[email protected]>

.github/ISSUE_TEMPLATE/project_thread.md

@@ -12,10 +12,12 @@ title: "<Replace this with actual title>"
 ## Documents
 
 <!-- Please link to these documents once they are written. -->
+<!-- Once a document is merged, please update the link to point at the published document on our documentation site: https://docs.openverse.org/projects/index.html -->
 
 - [ ] Project Proposal
 - [ ] Implementation Plan(s)
 
+
 ## Milestones/Issues
 
 <!-- Link to all the milestones or issues that must be completed as part of this project, typically those created after implementation planning but occasionally existing ones. -->