Improve JIRA development panel docs for configuration

There is some confusion with the configuration of the JIRA development in the docs. There are some customer ZD tickets (including https://gitlab.com/gitlab-org/gitlab-ee/issues/7399), and also some comments on the page itself (https://docs.gitlab.com/ee/integration/jira_development_panel.html).

Let's use this issue to improve the docs. In particular, I propose we have instructions and examples that are listed for each of these four scenarios:

• GitLab self-hosted, JIRA Server
• GitLab.com, JIRA Server
• GitLab self-hosted, JIRA Cloud
• GitLab.com, JIRA Cloud