build docs on configuration need an update

https://bazel.build/docs/build#build-config-cross-compilation talks about the target configuration and host configuration. It should be updated to discuss the exec configuration and perhaps also the role of toolchains. `--distinct_host_configuration` should not be mentioned, since it is [moribund](https://github.com/bazelbuild/bazel/commit/78d0fc9ff1d2f22005ddfce43384e35fbac338cb).

Read the linked Build configurations section in the Bazel documentation. Update its discussion of target and host configurations to cover the exec configuration and the role of toolchains, and remove the reference to --distinct_host_configuration. Done means the page reflects the current configuration model without mentioning that moribund option.

Written by the indexing model from the issue text.