Page: make mainContainerId prop required

[tlabaj]

The mainContainerId prop should be required for the skip to content to function as expected.

cc @seanforyou23

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.