[docs] add BoxLink component, use it on EAS introduction pages

[Simek]

Why

Currently, some pages server purpose as a section hubs. They do not include much content and offer users selected set of navigation options. Those page probably will not include upcoming Next/Previous Page buttons, so I thought it would be nice to improve those navigation options appearance, because otherwise, the pages will will a bit blank.

How

This PR adds the basic BoxLink link component, which has been designed to replace in-content navigation parts.

The new component has been used on the EAS Introduction pages for the whole service as-well-as for the Build and Submit sections.

Test Plan

The changes has been tested by running docs website locally.

Preview

Checklist

• Documentation is up to date to reflect these changes (eg: https://docs.expo.dev and README.md).
• This diff will work correctly for expo build (eg: updated @expo/xdl).
• This diff will work correctly for expo prebuild & EAS Build (eg: updated a module plugin).

[jonsamp]

One optional thing, and one thing we might look at. Otherwise this is looking good. After whatever changes you see fit, we should merge it!