feat: add backstage plugin page to the integrations category

[namkyu1999]

What this PR does / why we need it:

I added backstage plugin page to the integrations category.

• backstage plugin link: https://github.com/litmuschaos/backstage-plugin
• npm link: https://www.npmjs.com/package/backstage-plugin-litmus

Checklist:

• [ ] Fixes #
• [ ] Signed the commit for DCO to be passed
• [ ] Labelled this PR & related issue with documentation tag

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Name	Status	Preview	Comments	Updated (UTC)
litmus-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Oct 6, 2023 8:02am