♻️ Refactoring and cleaning the forge directory

[qd-qd]

This pull request contains several commits that refactor and clean the repository. The pull request is mostly focused on the forge directory.

Here is a summary of the changes:

• Added a .gitignore file to ignore automatically generated files during tests.
• Improved the main documentation of the repository by adding an installation process for Sage, Hardhat, and Forge.
• Fixed an error in the FCL_elliptic.t.sol test by passing the -f flag to the rm command.
• Linted Solidity files, excluding the Hardhat directory as it is a legacy project.
• Removed unused files.
• Replaced the homemade Base64 implementation with OpenZeppelin's Base64.sol library.