Fix various broken links in documentation articles. (#362)

Author: atierian

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](03%20Commands%20and%20Subcommands.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 [Command and Subcommands](./CommandsAndSubcommands.md), 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](./05%20Validation%20and%20Errors.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 [Handling Transform Errors](./Validation.md#handling-transform-errors) 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](../Sources/ArgumentParser/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](../../Parsable%20Types/EnumerableFlag.swift).
 
 ```
 % 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](05%20Validation%20and%20Errors.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 [Validation and Errors](./Validation.md), but using a scripty style instead of the typical command. First, we define the options as a `ParsableArguments` type:
 
 ```swift
 struct SelectOptions: ParsableArguments {
@@ -51,7 +51,7 @@ print(chosen.joined(separator: "\n"))
 
 Manually parsing commands is a little more complex than parsing a simple `ParsableArguments` type. The result of parsing from a tree of subcommands may be of a different type than the root of the tree, so the static `parseAsRoot()` method returns a type-erased `ParsableCommand`.
 
-Let's see how this works by using the `Math` command and subcommands defined in [Commands and Subcommands](03%20Commands%20and%20Subcommands.md). This time, instead of calling `Math.main()`, we'll call `Math.parseAsRoot()`, and switch over the result:
+Let's see how this works by using the `Math` command and subcommands defined in [Commands and Subcommands](./CommandsAndSubcommands.md). This time, instead of calling `Math.main()`, we'll call `Math.parseAsRoot()`, and switch over the result:
 
 ```swift
 do {