v21.0.0
doc/dev/crimson: Add BalanceCPUCrimson with clarifications as per conversation/review
doc/dev/crimson: update index
doc/dev/crimson: Add initial BackfillMachine docs
doc: Fix many spelling errors
doc: Fix link and improve Crimson doc
doc/dev/crimson/pipeline.rst: simplify and update to reflect new stages
doc/dev/seastore.rst: add design implementation for osd shards change
doc/dev/crimson/seastore: Update SeaStore docs