[DOC-496][yugabyted] Updating yugabyted reference page

[gargsans-yb]

Summary:

• Adding new xcluster commands for adding/removing databases
• Organising start command flags
• Adding exmples for xcluster to add/remove databases.

Reviewers: nikhil

Subscribrs: gargsans-yb

[CLAassistant]

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2 out of 3 committers have signed the CLA.

:white_check_mark: ddhodge
:white_check_mark: gargsans-yb
:x: Sanskar Garg

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