Documentation bug: Link to debugging FUDC in non-SDK projects is dead

## Visual Studio Version

NA

## Summary

In https://github.com/dotnet/project-system/blob/main/docs/up-to-date-check.md there is a link to https://github.com/dotnet/project-system/blob/main/docs/fast-up-to-date-legacy.md which is a 404.

## Steps to Reproduce

1. Go to https://github.com/dotnet/project-system/blob/main/docs/up-to-date-check.md
2. Click the second [documentation](https://learn.microsoft.com/visualstudio/extensibility/visual-cpp-project-extensibility#incremental-builds-and-up-to-date-checks) link.

## Expected Behavior

An article on FUDC for legacy-projects

## Actual Behavior

404

## User Impact

I had to dig up the process for managing FUDC on old non-SDK projects here:
https://github.com/dotnet/project-system/blob/b536d25b38d1a7770fb983c65c89b826f5af7a83/docs/up-to-date-check.md