Support searching alias in Azure PowerShell online reference doc

## Description of the new feature

For instance, user searches `Get-AzAks` but it cannot be found in the latest Az version because it is alias of `Get-AzAksCluster`. Many cmdlets have the similar problem, and make audience confused. We need to:

- Add alias information on online help file
- Index alias and allow user to search it

## Proposed implementation details (optional)

The issue names no files or tests; start by locating the Azure PowerShell online help generation and search-indexing entry points. Use Get-AzAks and its Get-AzAksCluster alias as the example, and consider the work done when aliases appear in online help and searching an alias finds the corresponding cmdlet.

Written by the indexing model from the issue text.