Add preview site

[zhammer]

Adds a Backstage preview site that serves as a demonstration for how to install the SeatGeek Backstage Plugins Collection and a way to see our plugins in use in a toy application.

Scaffolding the Backstage repository also gives us access to some helpful plugin monorepo tooling from the Backstage scripts. E.g., now we can run yarn test to execute tests for all of our plugins from the root directory, which will be helpful in CI.

[zhammer]

:tada: This PR is included in version 1.0.0 :tada:

The release is available on npm package (@latest dist-tag)

Your semantic-release bot :package::rocket:

[zhammer]

:tada: This PR is included in version 1.0.0 :tada:

The release is available on @seatgeek/backstage-plugin-entity-scaffolder-content@1.0.0

Your semantic-release bot :package::rocket:

[zhammer]

:tada: This PR is included in version 1.0.0 :tada:

The release is available on @seatgeek/backstage-plugin-awards@1.0.0

Your semantic-release bot :package::rocket:

[zhammer]

:tada: This PR is included in version 1.0.0 :tada:

The release is available on @seatgeek/backstage-plugin-awards-backend@1.0.0

Your semantic-release bot :package::rocket:

[github-actions[bot]]

:tada: This PR is included in version 1.0.0 :tada:

The release is available on @seatgeek/backstage-plugin-aws-catalog-common@1.0.0

Your semantic-release bot :package::rocket:

[github-actions[bot]]

:tada: This PR is included in version 1.0.0 :tada:

The release is available on @seatgeek/backstage-plugin-aws-catalog-backend@1.0.0

Your semantic-release bot :package::rocket:

[github-actions[bot]]

:tada: This PR is included in version 1.0.0 :tada:

The release is available on @seatgeek/backstage-plugin-gitlab-catalog-backend@1.0.0

Your semantic-release bot :package::rocket: