Fixes several deprecated items in the them aligned to Hugo version 0.139

[Tristor]

There were errors when doing a Hugo build due to deprecations after updating to the latest release, currently version 0.139.

Specifically this resolves three issues:

1. Site.Social is deprecated in favor of Site.Params.Social
2. Page.NextPage and Page.PrevPage are deprecated in favor of Page.Next and Page.Prev
3. Config parameter paginate is deprecated in favor of paginator.pagerSize

With these changes, it builds cleanly without my other changes.

[Tristor]

@yoshiharuyamashita Can you please merge this?

I updated the examples and docs as well as part of this change.

Thanks