missing commands in the instructions for building from source

[szha]

Description

As part of #18487 the build from source instruction no longer includes the commands for building from source. Also, it doesn't document the usage of the cmake configs in https://github.com/apache/incubator-mxnet/tree/master/config

[leezu]

The documentation is contained in the files. The instructions read "Please see the cmake configuration files files for instructions on how to configure and build MXNet with cmake."

https://github.com/apache/incubator-mxnet/blob/19e373daac76b466cf11b5d31fa5d5e2eb518a21/config/linux.cmake#L18-L36

Including more detailed and version-specific guides could be achieved as part of #18666