No Main within Main

[taysea]

Please describe what the rule should do:

A single Main should be used on a page.

What category of rule is this? (place an "X" next to just one item)

[x] Warns about a potential error (problem)
[ ] Suggests an alternate way of doing something (suggestion)
[ ] Other (please specify:)

Provide 2-3 code examples that this rule will warn about:

<Main>
  some stuff
   <Main>
      more stuff
   </Main>
</Main>

Why should this rule be included in eslint-plugin-grommet?

Accessibility best practice

[taysea]

This may be difficult to flag for with eslint since components can be abstracted across files / components. However, jest-axe already has a rule that can flag for this in a jest test. Wondering if we could leverage that in a recommended testing setup.