HDDS-1881. Design doc: decommissioning in Ozone (apache#1196)

* Address comments from Anu
 * In-flight calculation is removed from the base model
 * Throttling is mentioned but will be discussed in a separated document
 * Model is improved to handle the over-replication in a specific way
 (to handle Nanda's use-case)
 * Stop conditions are defined independently from the model
 * Style of the table fragments are improved
* typo fix (thx to sodonnell)
* remove white spaces from the end of the lines
* example is restored
* minor whitespace improvement to support pdf conversion

Author: elek