docs: add migration guide for v1.6

[ShubhamPalriwala]

What does this PR do?

Adds Migration Guide for the feature #1997

Checklist

Required

• Filled out the "How to test" section in this PR
• Read How we Code at Formbricks
• Self-reviewed my own code
• Commented on my code in hard-to-understand bits
• Ran pnpm build
• Checked for warnings, there are none
• Removed all console.logs
• Merged the latest changes from main onto my branch with git pull origin main
• My changes don't cause any responsiveness issues

Appreciated

• If a UI change was made: Added a screen recording or screenshots to this PR
• Updated the Formbricks Docs if changes were necessary

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[github-actions]

Thank you for following the naming conventions for pull request titles! 🙏

[review-agent-prime]

apps/formbricks-com/app/docs/self-hosting/migration-guide/page.mdx

Adding a table of contents at the beginning of the document with links to each section can improve the readability and navigation of the document. This will allow users to quickly jump to the section they are interested in.
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    # Migration Guide

    - [v1.6](#v1.6)
    - [v1.2](#v1.2)
    - [v1.1](#v1.1)

    ## v1.6
    ...

git fetch origin && git checkout -b ReviewBot/Impro-m3mizww origin/ReviewBot/Impro-m3mizww

Adding a brief summary of the changes for each version at the beginning of each section can help users quickly understand what changes were made in that version. This can be especially helpful for users who are migrating from an older version and want to quickly understand what changes they need to be aware of.
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    ## v1.6

    In this version, we have made several stability improvements and added new features. We have also made changes to our environment variables.

    ...

git fetch origin && git checkout -b ReviewBot/Impro-cdjor1v origin/ReviewBot/Impro-cdjor1v

[mattinannt]

@ShubhamPalriwala thank you for the changes, looks great 😊