ManualCpmBiddingScheme: Added definition

[rgaritta]

Added ManualCpm definition for consistency with ManualCpcBiddingScheme and ManualCpvBiddingScheme.

[opbld30]

Docs Build status updates of commit b5de91e:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
Advertising/bingads-13/campaign-management-service/manualcpmbiddingscheme.md	:white_check_mark:Succeeded	View (=bingads-13)

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[rgaritta]

Disregard, will update in another commit.