Host build / testbench docs

[jajanusz]

There is almost no info about testbench in docs (only how to build it).
We need description telling what it is and how to use it.

[jajanusz]

Probably @singalsu is the best person to address this

[lgirdwood]

@jajanusz yeah I agree. @singalsu I think we need some simple introduction and instruction for using the testbench (with say the SRC), e.g. just think of realworld usecase and detail the steps.

[singalsu]

I have some user's guide text written for SRC. I can extract the test bench part into sof-docs pretty quickly. Also the rest would be useful to publish.

[lgirdwood]

@singalsu sounds good to me.

[singalsu]

I haven't had time for this but stay tuned.

[jajanusz]

@singalsu You can wait for refactor first, to not write it twice: thesofproject/sof#1229