doc: erasure code developer notes updates

Update the introduction to explain erasure code profiles. Remove
obsolete explanations about partial writes etc. Remove links to tickets
used during development. Update permalinks to be closer to
Firefly (v0.78).

Signed-off-by: Loic Dachary <loic@dachary.org>
doc: update jerasure plugin

* The parameters come from the erasure code profile
* Add a note about the upstream submodules gf-complete / jerasure

Signed-off-by: Loic Dachary <loic@dachary.org>
doc: pgbackend dev doc outdated notice

* Warn the reader that the implementation is ahead and may differ
* Update the links to the Firefly branch
* Remove links to issues used during development to avoid confusion

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