Jira integration: final split of docs

This merge request finishes the job of hacking doc/user/project/integrations/jira.md apart, and distributing the pieces and bits to the various pages in the Jira integration section.

It does NOT clean up the individual pages. I'm concerned about merge conflicts here, due to the size of this forklift, and I would prefer to finish cleaning up the individual pages in separate merge requests that are less vulnerable to conflict.

When this work lands, the main page for the Jira integration(s) will be https://docs.gitlab.com/ee/integration/jira/index.html, which focuses on explaining what the different integrations are, and how to choose the right one for your needs. Each integration is then broken out into its own page, with a major feature clump (issue management and interactions) moved to its own page:

• index.md an explanation of the different types of Jira integrations we offer and how to know which one to use
• connect-app.md the Atlassian app
• development_panel.md the development panel integration
• dvcs.md the DVCS integration
• issues.md issue management in the integration
• jira_cloud_configuration.md create an API token in Jira on Atlassian cloud
• jira_server_configuration.md jira server credentials

Known followups needed

• Clean up the individual pages (this may push to %13.12, unsure) #327784 (closed)
• Update links in the www-gitlab-com repo gitlab-com/www-gitlab-com!79848 (merged)
• Update the left navigation in the gitlab-docs repo gitlab-docs!1700 (merged)
• Start cleaning up the app information in the external repo #324964 (closed)

Related links

Related to #324250 (closed) which is the overarching work here