* Change to 'pip install .. --extra-index-url' for readthedocs
* Change to 'pip install .. --extra-index-url' for examples
* Change to 'pip install .. --extra-index-url' for remaining files
* Fix URL for ipex
* Add links for ipex US and CN servers
* Update ipex cpu url
* remove readme
* Update for github actions
* Update for dockerfiles
* Set BIGDL_IMPORT_IPEX default to true, i.e., auto import IPEX for XPU.
* Remove import intel_extension_for_pytorch as ipex from GPU example.
* Add support for bigdl-core-xe-21.