Adds `gu:tool` and `gu:application-repo` definitions to AWS tags glossary

[SHession]

What is being recommended?

This PR adds to the set of standard AWS tags to include gu:tool and gu:application-repo. Hopefully the text content is descriptive enough to explain their use.

What's the context?

Related to the tools audit work and this comment: https://github.com/guardian/service-catalogue/pull/1525#discussion_r2091521062

These new tags are intended to make it easier to map between the resources that make up a project.

Note

The recommendations in this repository are intended to share engineering best practices for all of Product & Engineering (P&E) in the open.

Some considerations:

• Should this really be an ADR in another repository?
• Have you sought review widely enough (Developer Experience, Heads of Engineering)?
• If it is security related, should you consult with Information Security?