Update install jira application steps

What does this MR do?

Adjustments to documentation based on our walkthrough of the steps to install GitLab for Jira Cloud application on self-managed GitLab instance.

Please check the comments under Changes for more details.

Related issues

The Customer was confused by Step 4 in the documentation (no actual action was needed)

(Internal Ticket)

Snippet from relevant Slack discussion

Hello team! I have a question regarding [Install the GitLab Jira Cloud application manually for self-managed instances](https://docs.gitlab.com/ee/integration/jira/connect-app.html#install-the-application-manually). I am confused with Step 4, which points to the Atlassian guide Step 3 Install and test your app, which is similar to our own guide (Step 5,6,7) . Is this step (4. Install the GitLab application from your self-managed GitLab instance) necessary? I’m afraid I could be missing something here .

Justin Ho 3 days ago Looking at the docs you linked, 5,6,7 are more of sub-points of 4. So your understanding is correct.

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have Developer permissions or higher:
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Do not add the feature, frontend, backend, ~"bug", or database labels if you are only updating documentation. These labels will cause the MR to be added to code verification QA issues.

When applicable:

• Update the permissions table.
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• Add the product tier badge accordingly.
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• Add/update the feature flag section.

Review checklist

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1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

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For suggestions that you are confident don't need to be reviewed, change them locally and push a commit directly to save others from unneeded reviews. For example:

• Clear typos, like this is a typpo.
• Minor issues, like single quotes instead of double quotes, Oxford commas, and periods.

For more information, see our documentation on Merging a merge request.

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