FIP-0086: Rename Step to Phase for consistency (#1062)

Previously, the F3 implementation used "phase" and "step" inconsistently
throughout the repository. For instance, the type name was referred to
as "phase" while field names were labeled "step".

Following internal discussions, the implementation standardised the use
"phase" across the codebase to align terminology. This commit updates
the FIP documentation to reflect the changes for a consistent
terminology between the implementation and documentation.

Note, these changes are purely mechanical and do not alter any
functionality or behaviour.

Relates to filecoin-project/go-f3#634