Clean urls in the sidebar

[easeev]

Feature request

Provide suffix options for items the sidebar in plugin-clean-urls.

What problem does this feature solve?

Consistency in linking. Currently, sidebar items have .html suffix by default.

[luaz]

Using the following markdown link will still refer to .html and trailing slash

[Privacy with MD](./privacy.md) -> `/privacy.html`

[About with MD](./about/README.md) -> `/about/`

[easeev]

Sidebar links look good (/foo/bar/) while using normalSuffix: '/' but still contain .html while using normalSuffix: '' (/foo/bar.html).

@shigma any updates on this?