Update AI Gateway quickstart with improved structure and steps

[selectiveduplicate]

Purpose

This PR brings the updates made in a past PR that was overwritten in a recent restructuring.

Checklist

• Verified that llms.txt (located at en/docs/llms.txt) is updated for AI readiness content.
• Ensured meaningful alt text for images and that the information contained in an image also appears in text so the relevant info exists in the body of the document.
• Added or updated frontmatter for the respective pages.

Goals

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Approach

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User stories

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Release note

Brief description of the new feature or bug fix as it will appear in the release notes

Documentation

Link(s) to product documentation that addresses the changes of this PR. If no doc impact, enter “N/A” plus brief explanation of why there’s no doc impact

Training

Link to the PR for changes to the training content in https://github.com/wso2/WSO2-Training, if applicable

Certification

Type “Sent” when you have provided new/updated certification questions, plus four answers for each question (correct answer highlighted in bold), based on this change. Certification questions/answers should be sent to certification@wso2.com and NOT pasted in this PR. If there is no impact on certification exams, type “N/A” and explain why.

Marketing

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Automation tests

• Unit tests

  Code coverage information

• Integration tests

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Security checks

• Followed secure coding standards in http://wso2.com/technical-reports/wso2-secure-engineering-guidelines? yes/no
• Ran FindSecurityBugs plugin and verified report? yes/no
• Confirmed that this PR doesn't commit any keys, passwords, tokens, usernames, or other secrets? yes/no

Samples

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Related PRs

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Migrations (if applicable)

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Test environment

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Learning

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[coderabbitai]

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_{Comment @coderabbitai help to get the list of available commands.}

[Krishanx92]

Are we going to remove proxy deploy and invoke part from QSG?