v21.3.0
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:crimson-dev: add RANDOM_BLOCK_SSD usage example, fix indentation
doc/dev/crimson/pipeline.rst: simplify and update to reflect new stages
crimson/os/seastore: auto-tune cleaner gc segment pick under random-write
doc/dev/crimson/seastore: Update SeaStore docs
doc/dev/crimson/seastore_laddr.rst: add descriptions about temp recovering objects
