handbook: Add `Getting cross-team feedback on RFC` section

Created by: valerybugakov

Initially, I thought it would be great to have a separate board for RFC tracking, as we discussed briefly with @pdubroy.

On the other hand, we can leverage issue labels to show only RFCs, which will benefit from a cross-team review. It will mostly help other teams to find RFCs to review on our board (we can pin board-link with RFC filter in query params in a #frontend-platform-rfc-feed channel). Also, we won't need to move the RFC issue on two boards.

Let me know what you think about it.