Add initial customer limit mutation docs

[tpitale]

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This update adds documentation for a new customer limit mutation specifically for controlling the per-metric cardinality limit.

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[rhetoric101]

@tpitale I wasn't able to get the mutation to pass a linter, so I asked ChatGPT for some tips. I also updated the placeholders to SNAKE_CASE per our style guide. How does this look to you?

[tpitale]

Hey @rhetoric101 Sorry for the delay, I was out sick for a few days. Let me take a look and make some updates. Thanks!

[tpitale]

And yeah, this is the only mutation for now. But there will be a second one in short order to expire as well.

Otherwise, this looks great, thank you for the updates! Ready to merge when you are.

[rhetoric101]

@tpitale thanks for adding this!