[otbn] Move loop nesting into programmers guide
[otbn] Move loop nesting into programmers guide

The "Loop nesting" section in the OTBN documentation is targeting
programmers. Move it from the "Processor state" section to the
"Programmers guide" section.

Signed-off-by: Philipp Wagner <5a4b10454c30419e54a1570be358a516ff84bb63@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan