ErasureCode: doc updates

* Update to the current state of the ghobject implementaiton and the fact
  that they encode the shard_t Although the pool also contains the shard
  id, it is less relevant to understand the implementation.

* Update with the erasure code plugin infrastructure and the example
  plugin now in master.

* Move jerasure to a separate page to be expanded and link it from the
  toc

* Kill the partial read and writes notes as it will probably not be
  implemented in the near future. Kill some of the notes because they
  are no longer relevant.

* Add a definition for "chunk rank"

* Reword, update schemas, fix typos.

Signed-off-by: Loic Dachary <loic@dachary.org>