docs: removed navigation title from sidebar to docs dropdown

[iamayushdas]

Please answer these questions before submitting a pull request, or your PR will get closed.

Why submit this pull request?
To remove navigation title from sidebar to docs dropdown

• Bugfix
• New feature provided
• Improve performance
• Backport patches

What changes will this PR take into?

Please update this section with detailed description.

Related issues

fix/resolve #360 from apisix-website

Checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first

[netlify]

✔️ Deploy Preview for apisix-dashboard ready!

🔨 Explore the source changes: 85d6991

🔍 Inspect the deploy log: https://app.netlify.com/sites/apisix-dashboard/deploys/60bd78ffd7c4600007bc19a9

😎 Browse the preview: https://deploy-preview-1936--apisix-dashboard.netlify.app/