Index toc rendering issue

[Roiocam]

Fixes apache/pekko#975 (comment)

We can remove those element not belong the selected group.

[Roiocam]

This PR hides the element when toc header link is not the current group, I have verified this on my laptop.

But after PR #86 and #84, this plugin won't render the navigator anymore, because we put these render scripts to CDN,

you could verify this PR following this way:

1. checkout to the latest pekko project
2. nothing to do and then clean up and docs/paradoxBrowse, which means compile doc with the paradox
3. use this PR script to replace docs/target/paradox/site/main/assets/javascripts/groups.js
4. open the browser and refresh pekko doc page

Select Current Group to Java

Select Current Group to Scala

[Roiocam]

cc @He-Pin @mdedetrich @pjfanning @samueleresca

[He-Pin]

thanks, lgtm, but there is conflict. @Roiocam

[Roiocam]

close and reopen in the right repo: apache/pekko-site#72