docs: Modernize diagram in signatures.md

[gaureshpai]

Description

While reviewing through the documentation, I noticed that the diagram in the file signatures.md was broken, showing ASCII incompatible box drawing characters. This PR replaces those characters with a modern text-based version using ASCII characters.

Type of change

Please delete options that are not relevant.

• Documentation update

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Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

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Pull Request Title

docs: Modernize diagram in signatures.md