CG-10508: Docs explain differences between SourceFile and File types

[jemeza-codegen]

Motivation

SourceFile is currently not mentioned in the Building with Codegen Section

Content

Add content explaining the differences between SourceFile and File

• I have added tests for my changes (N/A)
• I have updated the documentation or added new documentation as needed
• I have read and agree to the Contributor License Agreement

[linear]

CG-10508 Get `File.get_symbol(...)` in docs

• Missing File.get_class, File.get_symbol, etc.
  • Explain File vs. SourceFile
• Add these in!
• Edit the Files and Directories guide

[jayhack]

@jemeza-codegen can we make it more obvious that using raw File is the minority case, and the majority of the time you are iterating over SourceFile?

[CLAassistant]

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