docs: there should be documentation aimed at Ti{DB|KV|Flash} devs

[nrc]

As a TiKV dev wanting to use TiUp to test my code, there is no documentation for me.

[lonng]

@nrc Are you using tiup playground to test your code? If so, can tiup playground --kv.binpath /path/to/debug/tikv-server solve your problem. We will sort our English documentation in Q4.

[lucklove]

We will introduce this in #840