Add Heimdall summary (#2830)

Used by the CORE SRE team Heimdall product to give
quick overview of all the HashiCorp repositories.
Initially this just adds some very basic information
about this repository. Could be expanded if we
expand the use of Heimdall.

META.d/_summary.yaml

@@ -0,0 +1,8 @@
+---
+schema: 1.1
+partition: secure
+category: product
+summary:
+  owner: team-boundary
+  description: The repository holding the Boundary OSS backend code and releases
+  visibility: external