[Feature] Add Basic DocC Documentation Support

Sources/Markdown/Markdown.docc/Markdown.md

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+# ``Markdown``
+
+Swift `Markdown` is a Swift package for parsing, building, editing, and analyzing Markdown documents.
+
+## Overview
+
+The parser is powered by GitHub-flavored Markdown's [cmark-gfm](https://github.com/github/cmark-gfm) implementation, so it follows the spec closely. As the needs of the community change, the effective dialect implemented by this library may change.
+
+The markup tree provided by this package is comprised of immutable/persistent, thread-safe, copy-on-write value types that only copy substructure that has changed. Other examples of the main strategy behind this library can be seen in Swift's [lib/Syntax](https://github.com/apple/swift/tree/master/lib/Syntax) and its Swift bindings, [SwiftSyntax](https://github.com/apple/swift-syntax).
+
+## Topics 
+
+### Essentials
+
+- ``Markup``
+- ``MarkupChildren``
+- ``ChildIndexPath``
+- ``TypedChildIndexPath``
+- ``DirectiveArgument``
+- ``DirectiveArgumentText``
+- ``Document``
+- ``LiteralMarkup``
+- ``PlainTextConvertibleMarkup``
+
+### Markup Types
+
+- <doc:BlockMarkup>
+- <doc:InlineMarkup>
+- ``Aside``
+
+### Infrastructure
+
+- <doc:Infrastructure> 
+
+### Visit Markup
+
+- <doc:VisitMarkup> 
+- <doc:FormatterAndOptions>

Sources/Markdown/Markdown.docc/Markdown/BlockMarkup.md

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+# Markup Block Nodes
+
+## Topics
+
+### Block Container Blocks
+- ``BlockDirective``
+- ``BlockQuote``
+- ``CustomBlock``
+- ``ListItem``
+- ``OrderedList``
+- ``UnorderedList``
+
+### Inline Container Blocks
+- ``Paragraph``
+
+### Leaf Blocks
+- ``Heading``
+- ``HTMLBlock``
+- ``ThematicBreak``
+- ``CodeBlock``
+
+### Tables
+
+- ``Table``
+- ``TableCellContainer``
+
+## See Also
+- <doc:BlockDirectives> 

Sources/Markdown/Markdown.docc/Markdown/FormatterAndOptions.md

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+# Formatter and Options
+
+## Topics
+
+### Formatter
+
+- ``MarkupFormatter``
+
+### Options
+
+- ``MarkupDumpOptions``

Sources/Markdown/Markdown.docc/Markdown/Infrastructure.md

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+# Infrastructure
+
+## Topics
+
+### Replacement
+
+- ``Replacement``
+
+### Source
+
+- ``SourceLocation``
+- ``SourceRange``

Sources/Markdown/Markdown.docc/Markdown/InlineMarkup.md

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+# Markup Inline Nodes
+
+## Topics
+
+### Inline Container
+- ``Emphasis``
+- ``Image``
+- ``Link``
+- ``Strikethrough``
+- ``Strong``
+
+### Inline Leaves
+- ``CustomInline``
+- ``InlineCode``
+- ``InlineHTML``
+- ``LineBreak``
+- ``SoftBreak``
+- ``SymbolLink``
+- ``Text``

Sources/Markdown/Markdown.docc/Markdown/VisitMarkup.md

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+# Visiting Markup
+
+## Topics
+
+### Vistor
+
+- ``MarkupVisitor``
+
+### Walker
+
+``MarkupWalker`` is a default implementation for ``MarkupVisitor``.
+
+- ``MarkupWalker``
+
+### Rewriter
+
+- ``MarkupRewriter``