Restructure introduction to GitLab <->Jira integration

Problem to solve

The current introduction to GitLab<->Jira integration does not present all options available. This may result in customer dissatisfaction because the integration does not meet their expectations.

Proposal

Rewrite and restructure the introduction to GitLab<->Jira integration to include all supported options, and direct readers to the appropriate instructions. The introduction should make the following clear to the reader:

• Available supported integration options.
• Features available via each option.
• Prerequisites of each option.

This issue replaces gitlab-foss#67310 (moved).