Add subscript to issue templates

[danielgblanco]

Description

As part of https://github.com/open-telemetry/sig-end-user/issues/11, this adds a subscript paragraph to all issue templates to help end-users and other community members interact with issues.

This change is accompanied by the changes in the maintainer guide proposed in https://github.com/open-telemetry/community/pull/2778, and the changes to the website in https://github.com/open-telemetry/opentelemetry.io/pull/7088 giving users more information about issue participation

The major caveat of this PR is the necessity of using the dropdown element in issue forms. As covered in this feedback discussion topic, it is not possible to create markdown elements that render on the resulting issue body. As such, the only possibilities are a checkbox, a dropdown, or some form of pre-filled input text that would be even more confusing.

Working version can be seen in https://github.com/danielgblanco/opentelemetry-collector-contrib/issues/6 with the issue form being rendered as https://github.com/danielgblanco/opentelemetry-collector-contrib/issues/new?template=bug_report.yaml