move SonataBlockBundle rendering flow documentation to sonata documentation

[dbu]

we have a quite good documentation of the process of rendering a block, as well as handling caching with blocks, in the cmf docs. we should instead move that doc to the sonata documentation and link it from our documentation.

see also https://github.com/symfony-cmf/symfony-cmf-docs/pull/282/files#r6716180 - adding a flow diagram would be even better.

[lsmith77]

do we still want to do this?