doc: deprecate EC plugins and techniques

doc: deprecate EC plugins and techniques

We want to reduce the number of EC plugins and techniques we support
in order to focus dev and test effort on the ones that are most
useful.

We are deprecating the following plugins and techniques in Umbrella,
and dropping support for them in the V release:
* shec
* clay
* all non-reed_sol_van jerasure techniques

This commit updates the documentation to reflect these changes.

Signed-off-by: Jamie Pryde <jamiepry@uk.ibm.com>