docs: point --help and the man page at the new overlay workflow

Both listed check and new-overlay among the commands but neither told a
reader how they fit together, which is the part that makes them useful. The
overlay sections now carry the four-step loop -- new-overlay, edit, check,
run -- and state what check does not do, since its name invites a stronger
reading than it earns.

Also documents what the format now permits: citations and layer optional, the
signed Easter_offset, the negative nth counting from the end of the month,
and the legal values of each of the six required fields, which previously
appeared only in the shipped example.

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