Move social into Render Engine `site_vars`

While I don't expect people to build with it all that much. It would be help as a way to shortcut some code to include social links in site_vars instead of themes.

Example:
```markdown
You can message me on [mastodon]({{social['mastododon']}})
```

vs.

```markdown
You can message me on [mastodon]({{theme.social['mastodon']}})
```

This starts to set a precedent on when [things should go in site_vars and when things should be theme specific]()(WIKI LINK TBD).

Start by locating the Render Engine code that exposes theme.social and site_vars, then compare how templates resolve each variable. Confirm where social links should live, update the relevant theme and site-variable handling, and verify that the example template can use site_vars without relying on theme.social.

Written by the indexing model from the issue text.