chore: add mainnet execution simulation guide

[ryanwaits]

What does this PR do?

• add a guide for mainnet execution simulation (mxs)

Reference
#985

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Name	Status	Preview	Comments	Updated (UTC)
hiro-systems-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 7, 2025 1:20pm

[hugoclrd]

Thanks @ryanwaits!
Left a couple of comments.

And before ## Limitations, can we add a section explaining how to set up an API key for mainnet execution simulation.

Basically, users need to set up an ENV var named HIRO_API_KEY=<api_key> with an API key generated on the Platform (could link to /stacks/api-keys or repeat the callout "Where can I get an API key?")

We also need to specify that currently the API key is only used for MXS (and other clarinet features don't use it).
The reason for this limitation is quite technical, we had to implement a custom HTTP client for MXS, and this client uses the API key. But other features don't use this client. Users probably don't need to know about that

[hugoclrd]

lgtm! thanks @ryanwaits 🙌