docs: Remove CI/CD overview link

[mmaslov007]

What changed

• Removed the redundant CI/CD Overview docs page so the CI/CD sidebar item behaves as an expandable category.
• Removed the Third-Party Integrations landing-page card that linked to the removed overview page.
• Left the CI/CD category metadata and Terraform child page in place.

Why

The CI/CD section was the only Third-Party Integrations top-level category with a direct overview-page link, making it behave differently from the other integration categories.

Fixes #7454.

Validation

• make install-hooks
• npx prettier docs/third-party-integrations/index.mdx --write
• npx prettier docs/third-party-integrations/index.mdx --check
• npm run build

Note: make lint currently stops on pre-existing Prettier drift across the docs tree before it reaches the build step. The direct Docusaurus build completed successfully; it still reports one unrelated existing broken anchor from /deployment-self-hosting/core-configuration/running-flagsmith-on-flagsmith to /administration-and-security/access-control/oauth#github.

[vercel]

@mmaslov007 is attempting to deploy a commit to the Flagsmith Team on Vercel.

A member of the Team first needs to authorize it.

[gemini-code-assist]

Code Review

This pull request removes the "CI/CD Overview" documentation page and its corresponding card from the third-party integrations index page. Additionally, it formats the Grafana & Prometheus integration link for better readability. I have no feedback to provide as there are no review comments or issues identified.