- Adds a swift package manager command plugin called
  GenerateManualPlugin. The plugin can be invoked from the command line
  using `swift package experimental-generate-manual`. The plugin is
  prefixed for now with "experimental-" to indicate it is not mature and
  may see breaking changes to its CLI and output in the future. The
  plugin can be can be used to generate a manual in MDoc syntax for any
  swift-argument-parser tool that can be executed via
  `tool --experimental-dump-info`.
- The plugin works by converting the `ToolInfoV0` structure from the
  `ArgumentParserToolInfo` library into MDoc AST nodes using a custom
  (SwiftUI-esk) result builder DSL. The MDoc AST is then lowered to a
  string and written to disk.
- The MDoc AST included is not general purpose and doesn't represent the
  true language exactly, so it is private to the underlying
  `generate-manual` tool. In the future it would be interesting to
  finish fleshing out this MDoc library and spin it out, however this is
  not a priority.
- Next steps include:
  - Improving the command line interface for the plugin.
  - Adding support for "extended discussions" to Commands and exposing
    this information in manuals.
  - Further improve the escaping logic to properly escape MDoc macros
    that might happen to appear in user's help strings.
  - Ingesting external content a-la swift-docc so the entire tool
    documentation does not need to be included in the binary itself.
  - Bug fixes and addressing developer/user feedback.

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@rauhul