feat: Highlight or add left border to active navbar subitem

[janvi01]

Just like we navigate to /download, the download nav item gets a left border for distinction. The same can apply to nav sub-items.

For example - when we navigate to /environment under documentation in the nav bar, the environment UI should also change similarly.

[janvi01]

@SableRaf Kindly look, I would like to work on this.

[Ankitapanda25]

I have been working on react.js for quite some time, I would like to work on this

[gunjjoshi]

So, you mean, that the navs under the "Table of Contents" section must display the same left border upon selection, right ?