Add plausible analytics to home page

[MridulS]

Yes, anyway we need to do a manual build and push after this to networkx/networkx.github.io which hosts the raw github pages deployment. I can do that after merging this

[MridulS]

I'll go ahead and self merge this one and see if everything works.

If this works fine, we can also add plausible to our docs.

[MridulS]

Looks like its working :)