Add Incident Response to billing docs

[amlakhanpal]

What does this PR do? What is the motivation?

Merge instructions

Merge readiness:

• Ready for merge

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Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/amlakhanpal/incident-response-billing/account_management/billing/incident_response

[aflinois]

Edits suggeted:

"Each product bills by seat, meaning every user who performs qualifying actions requires a seat license. For detailed pricing information, see the Incident Response pricing page."

in the "What is a seat?" section can we bold some of the words like on-call seat, incident management seat and incident response seat?

"Claiming a Seaet" --> typo to fix

"How Billing Works" --> can we add what happens when

• a user claims a seat in a give month --> the seat is charged M2M forever until it is removed
• a user is removed in a given month --> the seat is still charged that month but not the following month

[cswatt]

Looks good overall! Just some nitpicky things