docs: Add guidelines for masthead launch icon location

[kaylachumley]

We need some design guidelines recommendations for the chatbot "launch" icon location to be in the masthead. This is an opt in feature so the users can choose if they want to use the masthead or bottom right floating icon on the screen - Will need a core follow up issue to add demo and docs content with demo

@edonehoo when you are ready to work on this, reach out to @kaylachumley for guidance on the Ai icon to use in the masthead for the design guidelines screenshots and or the demo. the ai enablement team has been working on a new icon set for ai experiences so itll probably be a icon with a sparkle or robot of sort

examples:

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Jira Issue: PF-3545