Update documentation cross-links (#383)

Author: natecook1000

Sources/ArgumentParser/Documentation.docc/ArgumentParser.md

@@ -37,26 +37,12 @@ the `ArgumentParser` library parses the command-line arguments,
 instantiates your command type,
 and then either calls your `run()` method or exits with a useful message.
 
-```
-$ repeat hello --count 3
-hello
-hello
-hello
-$ repeat --help
-USAGE: repeat [--count <count>] <phrase>
-
-ARGUMENTS:
-  <phrase>                The phrase to repeat.
-
-OPTIONS:
-  --count <count>         The number of times to repeat 'phrase'.
-  -h, --help              Show help for this command.
-$ repeat --count 3
-Error: Missing expected argument 'phrase'.
-Help:  <phrase>  The phrase to repeat.
-Usage: repeat [--count <count>] <phrase>
-  See 'repeat --help' for more information.
-```
+![The output of the Repeat command, declared above.](repeat.png)
+
+#### Additional Resources
+
+- [`ArgumentParser` on GitHub](https://github.com/apple/swift-argument-parser/)
+- [`ArgumentParser` on the Swift Forums](https://forums.swift.org/c/related-projects/argumentparser/60)
 
 ## Topics
 

Sources/ArgumentParser/Documentation.docc/Articles/CustomizingHelp.md

@@ -76,7 +76,7 @@ OPTIONS:
 
 ## Customizing Help for Commands
 
-In addition to configuring the command name and subcommands, as described in [Command and Subcommands](./CommandsAndSubcommands.md), you can also configure a command's help text by providing an abstract and discussion.
+In addition to configuring the command name and subcommands, as described in <doc:CommandsAndSubcommands>, you can also configure a command's help text by providing an abstract and discussion.
 
 ```swift
 struct Repeat: ParsableCommand {

Sources/ArgumentParser/Documentation.docc/Articles/DeclaringArguments.md

@@ -199,7 +199,7 @@ struct Example: ParsableCommand {
 }
 ```
 
-Throw an error from the `transform` function to indicate that the user provided an invalid value for that type. See [Handling Transform Errors](./Validation.md#handling-transform-errors) for more about customizing `transform` function errors.
+Throw an error from the `transform` function to indicate that the user provided an invalid value for that type. See <doc:Validation> for more about customizing `transform` function errors.
 
 ## Using flag inversions, enumerations, and counts
 
@@ -255,7 +255,7 @@ struct Example: ParsableCommand {
 }
 ```
 
-The flag names in this case are drawn from the raw values — for information about customizing the names and help text, see the  [`EnumerableFlag` documentation](../../Parsable%20Types/EnumerableFlag.swift).
+The flag names in this case are drawn from the raw values — for information about customizing the names and help text, see the  ``EnumerableFlag`` documentation.
 
 ```
 % example --in-memory-cache --pink --silver

Sources/ArgumentParser/Documentation.docc/Articles/ManualParsing.md

@@ -10,7 +10,7 @@ For most programs, calling the static `main()` method on the root command type i
 
 For simple Swift scripts, and for those who prefer a straight-down-the-left-edge-of-the-screen scripting style, you can define a single `ParsableArguments` type to parse explicitly from the command-line arguments.
 
-Let's implement the `Select` command discussed in [Validation and Errors](./Validation.md), but using a scripty style instead of the typical command. First, we define the options as a `ParsableArguments` type:
+Let's implement the `Select` command discussed in <doc:Validation>, but using a scripty style instead of the typical command. First, we define the options as a `ParsableArguments` type:
 
 ```swift
 struct SelectOptions: ParsableArguments {