Move illink documentation to runtime's documentation location (#78520)

Fix links to point to new locations

src/tools/illink/docs/design/constant-propagation-and-branch-removal.md → docs/design/tools/illink/constant-propagation-and-branch-removal.md

@@ -12,7 +12,7 @@ Method can return constant value if its code will always return the same value (
     public bool Is32Bit { get => false; }
 ```
 
-On 64bit platforms the property is compiled with constant value, and ILLInk can determine this. It's also possible to use substitutions to overwrite method's return value to a constant via the [substitutions XML file](../data-formats.md#substitution-format).
+On 64bit platforms the property is compiled with constant value, and ILLInk can determine this. It's also possible to use substitutions to overwrite method's return value to a constant via the [substitutions XML file](../../../tools/illink/data-formats.md#substitution-format).
 
 If such method is used in another method and it influences its return value, it can mean that the caller method will itself always return the same value. For example:
 

src/tools/illink/docs/README.md → docs/tools/illink/README.md

@@ -13,5 +13,5 @@ View the official trimming docs at [docs.microsoft.com](https://docs.microsoft.c
 - [Authoring Custom Steps](custom-steps.md)
 
 ## Design Docs
-- [Trimmer Steps Explained](design/steps-explained.md)
-- [Reflection Handling](design/reflection-flow.md)
+- [Trimmer Steps Explained](../../design/tools/illink/steps-explained.md)
+- [Reflection Handling](../../design/tools/illink/reflection-flow.md)

src/tools/illink/docs/custom-steps.md → docs/tools/illink/custom-steps.md

@@ -54,4 +54,4 @@ The common operation in the custom steps is to run additional checks on the IL t
 required constraints. The outcome of such operation is then usually some indication to the end user about
 the incompatible code. This can be done by reporting the error or warning message using `context.LogMessage`
 method. It's good practice to use error or warning code which does not conflict with existing codes as
-listed [here](/docs/error-codes.md).
+listed [here](error-codes.md).

src/tools/illink/docs/illink-options.md → docs/tools/illink/illink-options.md

@@ -123,7 +123,7 @@ unreachable will be removed.
 
 ### Adding custom linker steps
 
-You can write [custom steps](/docs/custom-steps.md) for the linker and instruct
+You can write [custom steps](custom-steps.md) for the linker and instruct
 the linker to add them into its existing pipeline. To tell the linker where this assembly is
 located, you have to append its full path after a comma which separates the custom
  step's name from the custom assembly's path