[SwiftLexicalLookup][GSoC] Add a DocC article with description of SwiftLexicalLookup.

[MAJKFL]

This PR includes a DocC article with high-level description of SwiftLexicalLookup. We thought with @DougGregor it would be great to include it with the final GSoC submission.

[ahoppen]

Please double-check that this works but I think docc supports [!IMPORTANT], which would add some color here

Suggested change

	> Important:
	> [!IMPORTANT]

[MAJKFL]

Sadly it doesn't seem to work. It just creates a plain note and embeds [!IMPORTANT] in the rest of the text.

I've also tried [!IMPORTANT]:, **Important:** and **Important**:, but haven't had much luck with them either.

[ahoppen]

Oh, apparently the format is

> Important: Your text

Like here: https://github.com/swiftlang/swift-docc/blob/main/Sources/docc/DocCDocumentation.docc/adding-structure-to-your-documentation-pages.md?plain=1#L51

Rendered as seen here: https://www.swift.org/documentation/docc/adding-structure-to-your-documentation-pages

[MAJKFL]

Ok, I made it inline.

[DougGregor]

@swift-ci please test

[MAJKFL]

Also, the CI failed for macOS in a bunch of tests

Failed Tests (9):
  Swift(macosx-x86_64) :: ModuleInterface/features.swift
  Swift(macosx-x86_64) :: Parse/ConditionalCompilation/compiler.swift
  Swift(macosx-x86_64) :: Parse/ConditionalCompilation/language_version_explicit.swift
  Swift(macosx-x86_64) :: Parse/ConditionalCompilation/sequence_version.swift
  Swift(macosx-x86_64) :: Parse/features.swift
  Swift(macosx-x86_64) :: SILGen/specialize_attr.swift
  Swift(macosx-x86_64) :: SILOptimizer/pre_specialize-macos.swift
  Swift(macosx-x86_64) :: SILOptimizer/pre_specialize.swift
  Swift(macosx-x86_64) :: SILOptimizer/pre_specialize_layouts.swift

They don't seem to be related much with this PR though. Is it possible I forgot to add something important here with the documentation?

[DougGregor]

@swift-ci please test

[DougGregor]

@swift-ci please test Windows