Merge branch 'add-link-to-issue-boards-in-api-readme' into 'master'

Add a link to the Issue Boards API in main README

See merge request !7221

doc/api/README.md

@@ -23,6 +23,7 @@ following locations:
 - [Group Access Requests](access_requests.md)
 - [Group Members](members.md)
 - [Issues](issues.md)
+- [Issue Boards](boards.md)
 - [Keys](keys.md)
 - [Labels](labels.md)
 - [Merge Requests](merge_requests.md)