docs: move parameters config.md -> db_config.md

Address changes from picodata@c2ae3ade (24.7.0-1144)

Move the following parameters:
* `instance.memtx.checkpoint_interval` -> `memtx_checkpoint_interval`
* `instance.memtx.checkpoint_count` -> `memtx_checkpoint_count`
* `instance.iproto.max_concurrent_messages` -> `iproto_net_msg_max`