Document security considerations for the GitLab for Jira Cloud app

Add a section to the GitLab for Jira Cloud app that answers common security questions:

From @apham4

• How is the OAuth API read/write access used?
• What other access is included in the read/write access?
• How does OAuth for the GitLab for Jira app work?
• Are requests incoming from Jira limited to certain projects?
• Can Jira retrieve source code?
• What access does GitLab have to Jira?
• How is access to Jira granted for GitLab?

From @obicke

• Has GitLab completed or achieved one or more security certifications (regarding Jira)? If yes, which ones
• Does GitLab only have access to information contained in Jira? (not other Atlassian products) #396424 (comment 1314803463)
• Does the app store data in (i) the main product (Confluence/Jira/Slack), (ii) Amazon Web Services (AWS), (iii) Google Cloud Platform, or (iv) Microsoft Azure? If so which one? #396424 (comment 1314061726)
• Can customer admins or individual users control the app settings and access permissions to limit the data/information the app accesses/stores/sees? #396424 (comment 1314056721)
• Is the GitLab for Jira app easily uninstalled with minimal disruptions to the user experience? #396424 (comment 1314814000)