add docs for estimating gas limit within mxpy

[popenta]

Description of the pull request (what is new / what has changed)

Did you test the changes locally ?

• yes
• no

Which category (categories) does this pull request belong to?

• document new feature
• update documentation that is not relevant anymore
• add examples or more information about a component
• fix grammar issues
• other

[Copilot]

Pull Request Overview

This PR adds documentation for a new gas limit estimation feature in mxpy CLI. The feature allows automatic estimation of gas limits for transactions when a proxy URL is provided, with options to add safety margins.

Key Changes

• Documents gas limit estimation functionality available in mxpy version 11.1.0+
• Explains how to use gas limit multipliers for safety margins
• Provides examples for both per-transaction and global configuration options

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