dev/osd_internals,src/osd: add erasure_coding.rst and PGBackend.h

Signed-off-by: Samuel Just <sam.just@inktank.com>
M+K means M data + K parity and implies M chunks necessary to recover

Signed-off-by: Loic Dachary <loic@dachary.org>
fix typo

Signed-off-by: Loic Dachary <loic@dachary.org>
Add links to backfill related tracker items.

Signed-off-by: Loic Dachary <loic@dachary.org>