[SE-2763] Explain JIRA fields, task creation and JIRA tips

This fixes:

• #11 (closed), how to add a ticket and which fields to use
• #12 (closed), how to close a ticket
• #18 (closed), how to use JIRA, what do colors and icons mean, and what can be ignored

The onboarding course will mention this section when inviting the newcomer to take a first look at JIRA: onboarding-course!65 (merged)

How to test this MR:

• mainly check the markdown format (internal links). You can use Gitlab's preview
• check the formatting (not very consistent; I more-or-less wrote fields names as code, and references as italic)
• check that everything was addressed
• check that it's factually correct, and not too out of date
• check writing style

After merging this and onboarding-course!65 (merged), close the 3 issues mentioned above.