Make Jira app prerequisites more visible to readers

What does this MR do?

We've had some reviews in the Jira app that it doesn't work with self-managed instances https://marketplace.atlassian.com/apps/1221011/gitlab-com-for-jira-cloud?tab=reviews&hosting=cloud

Unfortunately we don't know the reason why for most of these reviews, but I'm wondering if users are missing some the prerequisites in our documentation. The way our docs are structured, it can easily be missed.

For Connect the GitLab for Jira Cloud app for self-managed instances:

1. Removed the prerequisite "GitLab.com must serve as a proxy for the instance." as this is really a note. I've moved this into the main body.
2. Moved the prerequisites under the main body of text
3. Changed prerequisites to a header
4. Changed the steps so users must check prerequisites are met before continuing.

For Install the GitLab for Jira Cloud app manually:

1. Changed prerequisites to a header
2. Changed the steps for both methods so users must check prerequisites are met before continuing.

Related issues

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