* Add experimental markdown output flag and pass it through to the convert feature flags

* Initial export of Markdown from Article

* Initial processing of a type-level symbol

* Adds symbol declarations and article reference links

* Output tutorials to markdown

* Be smarter about removing indentation from within block directives

* Baseline for adding new tests for markdown output

* Basic test infrastructure for markdown output

* Adds symbol link tests to markdown output

* Tutoorial code rendering markdown tests

* Adding metadata to markdown output

* Include package source for markdown output test catalog

* Output metadata updates

* Adds default availability for modules to markdown export

* Move availability out of symbol and in to general metadata for markdown output

* Refactor markdown output so the final node type is standalone

* Add generated markdown flag to render node metadata

* Only include unavailable in markdown header if it is true

* Initial setup of manifest output, no references

* Output of manifest / relationships

* Manifest output format updates

* Remove member symbol relationship

* More compact availability, deal with metadata availability for symbols

* Update tests so all inputs are locally defined

* Remove print statements from unused visitors

* Added snippet handling

* Remove or _spi-hide new public API

* Remove or _spi-hide new public API (part 2)

* Prevent recursion when the abstract of a link contains a link and we are rendering links-with-abstracts

* Test and temporary fix for colspan issue swiftlang/swift-markdown#238

* Remove separate SwiftDocCMarkdownOutput target

* Bump swift-markdown, remove temporary bug fix

* Updates to handle removed bundle parameter

* Extract common writer code into a helper function

* Clarify use of article.md in test catalog

* Deal with internal links with anchor tags

* Deal with unresolved symbol links

* Updates to improve public API / clarity of MarkdownOutputNode

* Improve public API of manifest, sort contents before writing to prevent unneccessary diffs on re-run

* Don't use unstructured `Data` for WritableMarkdownOutputNode

* Reduce public SPI surface around markdown writing

* Make relationship subtype a specific type rather than a string

* Make it clear that path component is a fallback title, harden linked list rendering

* Add todos and missing tests

* Move markdown output types from public SPI to package

* don't include scheme or host in documentation links

* Reformat multi-condition if statements

* remove whitespace-only change

* Remove unused declaration

* Remove redundant return types

* Remove MarkdownOutputNodeTranslator

* Add TODO for alternate declarations

* Add TODO for structural review

* Add stacks todo, set writable type back to package

* uri -> identifier

* Use RelationshipsGroup.Kind instead of creating a mirror type

* Lower access level of manifest, remove children method

* Restrict package level declarations to those actually used at package level

* Merging main

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Co-authored-by: Richard Turton <[email protected]>