Update developer docs to no longer support VCPKG_ROOT as a mechanism to set/find global versions of the vcpkg repo

[ahsonkhan]

Similar to Azure/azure-sdk-for-c#2404, consider updating AzureVcpkg.cmake to no longer support VCPKG_ROOT as a mechanism to set where the vcpkg registry is located locally.

azure-sdk-for-cpp/tools/AzureVcpkg.cmake

Lines 5 to 19 in d1b05c2

	# AUTO CMAKE_TOOLCHAIN_FILE:
	# User can call `cmake -DCMAKE_TOOLCHAIN_FILE="path_to_the_toolchain"` as the most specific scenario.
	# An env var VCPKG_ROOT or VCPKG_INSTALLATION_ROOT can be set to let Azure SDK to set the VCPKG toolchain automatically.
	# As the last alternative (default case), Azure SDK will automatically clone VCPKG folder and set toolchain from there.
	if(NOT DEFINED CMAKE_TOOLCHAIN_FILE)
	if(DEFINED ENV{VCPKG_ROOT})
	set(CMAKE_TOOLCHAIN_FILE "$ENV{VCPKG_ROOT}/scripts/buildsystems/vcpkg.cmake"
	CACHE STRING "")
	elseif(DEFINED ENV{VCPKG_INSTALLATION_ROOT})
	set(CMAKE_TOOLCHAIN_FILE "$ENV{VCPKG_INSTALLATION_ROOT}/scripts/buildsystems/vcpkg.cmake"
	CACHE STRING "")
	else()
	# Set AZURE_SDK_DISABLE_AUTO_VCPKG env var to avoid Azure SDK from cloning and setting VCPKG automatically
	# This option delegate package's dependencies installation to user.
	if(NOT DEFINED ENV{AZURE_SDK_DISABLE_AUTO_VCPKG})