add an option to render the navbar when `onlyCover` is true

[trusktr]

Summary

add an option to render the navbar when onlyCover is true

What kind of change does this PR introduce?

Feature

For any code change,

• [x] Related documentation has been updated if needed
• [ ] Related tests have been updated or tests have been added

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

Related issue, if any:

fixes https://github.com/docsifyjs/docsify/issues/1696

Tested in the following browsers:

• [x] Chrome
• [ ] Firefox
• [ ] Safari
• [ ] Edge
• [ ] IE

[vercel[bot]]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/docsify-core/docsify-preview/BCJ5sQJfwakGi1VCapq32HDvKK4P
✅ Preview: https://docsify-preview-git-onlycovernavbar-option-docsify-core.vercel.app

[codesandbox-ci[bot]]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 43a1bfe662ab7e32e76c1361de4675a8c9a32d8d:

Sandbox	Source
docsify-template	Configuration
nice-zhukovsky-m3jl7	Issue #1696
nice-dust-9ytri	Issue #1696

[trusktr]

This pull request is being automatically deployed with Vercel (learn more). To see the status of your deployment, click below or on the icon next to each commit.

mag Inspect: https://vercel.com/docsify-core/docsify-preview/5pGhPTDoqteu2Z5vcCXaqKtZkkvy white_check_mark Preview: https://docsify-preview-git-onlycovernavbar-option-docsify-core.vercel.app

It would be awesome if we could somehow specify Docsify options to use for the vercel previews, and have two previews: one set to preview the actual docs site works fine, and one set to preview config/feature changes.