Add Solidity require concept to Blockchain section.

[benpaymaster]

Description

This PR adds a new concept entry for the Solidity require statement under the Blockchain section.

The entry includes:

• Explanation of require
• Syntax
• Example of access control with a Bank contract
• Security considerations

This follows the Codecademy Docs structure for concepts, matching other subjects like JavaScript and Python.

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Issue Solved

Closes N/A

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Type of Change

• Adding a new entry
• Editing an existing entry (fixing a typo, bug, issues, etc)
• Updating the documentation

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Checklist

• All writings are my own.
• My entry follows the Codecademy Docs style guide.
• My changes generate no new warnings.
• I have performed a self-review of my own writing and code.
• I have checked my entry and corrected any misspellings.
• I have made corresponding changes to the documentation if needed.
• I have confirmed my changes are not being pushed from my forked main branch.
• I have confirmed that I'm pushing from a new branch named after the changes I'm making.
• I have linked any issues that are relevant to this PR in the Issues Solved section.

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