Docs page explaining how feature flags get billed

[andyvan-ph]

via @zlwaterfield

The biggest user confusion we get on the billing team is understanding how feature flags get billed and where their usage is from. I think we could have a good guide / docs that:
• explains exactly how we bill, similar to https://posthog.com/docs/feature-flags/cutting-costs and https://posthog.com/docs/feature-flags/common-questions, but more clear on every single thing that counts towards being billing
• describes how customers should think about flags on the client and the server
• talks about caching strategies to lower billable calls especially for SSR

(https://posthog.slack.com/archives/C07Q2U4BH4L/p1746527086294199?thread_ts=1746480186.417289&cid=C07Q2U4BH4L)

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