Add the word "release" to the glossary

[edwbuck]

In the glossary, the word "Release" is missing.

It's missing because there were two "release" definitions, each meaning wildly different things, depending on what context it was being used in.

1. It refers to a Helm Chart offering of the spire/helm-chart project, located in https://github.com/spiffe/helm-charts/releases.
2. It is a Helm Chart term, describing an instance of a Chart running in a Kubernetes cluster.

I feel that to coordinate on this term, we need to add it to the glossary with both definitions, and clarifying entries "project release" and "helm release" which only have their specific definition.

The remaining entries should be updated to align with whatever we add.