[New term] `Backstage` - Githubissues

This term is to give a small introduction/overview on Backstage project. Which is one of the CNCF Incubating maturity level project.
Why this term needs to be added? (briefly)
Since it's a part of CNCF so it has to be added in Glossary so that people can get an idea on Backstage project.

[X] The suggested term fits the cloud native definition or is essential to understanding cloud native terms. (We hope to avoid adding general or broad terminologies)
[X] There is no existing issue yet. (shift+click to check existing issues)
[X] The term has not already been declined in the past. (shift+click to check)
[X] The term does not already exist in the glossary as a completed term, nor does it exist in this directory as an incomplete term. (If the status of the term is Feedback Appreciated, consider updating the term.)
[X] I understand that maintainers will assess if the term should be part of the Glossary. (Please be patient)
[X] I want to work on this term and confirm that I meet the criteria as outlined in "who can contribute". (If not checked, maintainers will assign another volunteer)

Maintainers will assign a label after assessment procedure. (from triage/awaiting to triage/accepted or triage/not accepted)

A contributor can be assigned to this issue after the issue get the triage/accepted label. The assignee needs to check Style Guide.

cncf / glossary