Broken links in source-build documentation

[MichaelSimons]

There are several broken links in the source-build documentation that should be fixed.

From a recent PR:

  Raw Output:
  message:"Cannot reach https://canonical.com/blog/install-dotnet-on-ubuntu Status: 500" location:{path:"README.md" range:{start:{line:112 column:3} end:{line:112 column:64}}} severity:ERROR source:{name:"linkspector" url:"https://github.com/UmbrellaDocs/linkspector"}
  reviewdog: found at least one issue with severity greater than or equal to the given level: any
  Error: [Linkspector] reported by reviewdog 🐶
  Cannot reach https://packages.ubuntu.com/search?suite=default&section=all&arch=any&keywords=dotnet&searchon=names Status: 500
  
  Raw Output:
  message:"Cannot reach https://packages.ubuntu.com/search?suite=default&section=all&arch=any&keywords=dotnet&searchon=names Status: 500" location:{path:"README.md" range:{start:{line:112 column:67} end:{line:112 column:178}}} severity:ERROR source:{name:"linkspector" url:"https://github.com/UmbrellaDocs/linkspector"}
  Error: [Linkspector] reported by reviewdog 🐶
  Cannot reach https://github.com/dotnet/dotnet/blob/main/src/sdk/eng/pipelines/templates/stages/vmr-build.yml Status: 404
  
  Raw Output:
  message:"Cannot reach https://github.com/dotnet/dotnet/blob/main/src/sdk/eng/pipelines/templates/stages/vmr-build.yml Status: 404" location:{path:"Documentation/system-requirements.md" range:{start:{line:15 column:3} end:{line:15 column:104}}} severity:ERROR source:{name:"linkspector" url:"https://github.com/UmbrellaDocs/linkspector"}
  Error: [Linkspector] reported by reviewdog 🐶
  Cannot reach https://github.com/dotnet/sdk/blob/main/eng/pipelines/templates/stages/vmr-build.yml Status: 404
  
  Raw Output:
  message:"Cannot reach https://github.com/dotnet/sdk/blob/main/eng/pipelines/templates/stages/vmr-build.yml Status: 404" location:{path:"Documentation/ci-platform-coverage-guidelines.md" range:{start:{line:38 column:55} end:{line:39 column:94}}} severity:ERROR source:{name:"linkspector" url:"https://github.com/UmbrellaDocs/linkspector"}
  Error: [Linkspector] reported by reviewdog 🐶
  Cannot reach https://github.com/dotnet/sdk/blob/main/src/SourceBuild/content/global.json Status: 404
  
  Raw Output:
  message:"Cannot reach https://github.com/dotnet/sdk/blob/main/src/SourceBuild/content/global.json Status: 404" location:{path:"Documentation/VMR-re-bootstrapping.md" range:{start:{line:85 column:8} end:{line:85 column:98}}} severity:ERROR source:{name:"linkspector" url:"https://github.com/UmbrellaDocs/linkspector"}
  Error: [Linkspector] reported by reviewdog 🐶
  Cannot reach https://github.com/dotnet/sdk/blob/main/src/SourceBuild/content/global.json Status: 404
  
  Raw Output:
  message:"Cannot reach https://github.com/dotnet/sdk/blob/main/src/SourceBuild/content/global.json Status: 404" location:{path:"Documentation/VMR-re-bootstrapping.md" range:{start:{line:91 column:8} end:{line:91 column:98}}} severity:ERROR source:{name:"linkspector" url:"https://github.com/UmbrellaDocs/linkspector"}
  Error: [Linkspector] reported by reviewdog 🐶
  Cannot reach https://github.com/dotnet/sdk/blob/main/src/SourceBuild/content/eng/Version.Details.xml Status: 404
  
  Raw Output:
  message:"Cannot reach https://github.com/dotnet/sdk/blob/main/src/SourceBuild/content/eng/Version.Details.xml Status: 404" location:{path:"Documentation/VMR-re-bootstrapping.md" range:{start:{line:93 column:8} end:{line:93 column:118}}} severity:ERROR source:{name:"linkspector" url:"https://github.com/UmbrellaDocs/linkspector"}
  Error: [Linkspector] reported by reviewdog 🐶
  Cannot reach https://github.com/dotnet/sdk/blob/main/src/SourceBuild/content/eng/Versions.props Status: 404
  
  Raw Output:
  message:"Cannot reach https://github.com/dotnet/sdk/blob/main/src/SourceBuild/content/eng/Versions.props Status: 404" location:{path:"Documentation/VMR-re-bootstrapping.md" range:{start:{line:97 column:8} end:{line:97 column:108}}} severity:ERROR source:{name:"linkspector" url:"https://github.com/UmbrellaDocs/linkspector"}
  Error: [Linkspector] reported by reviewdog 🐶
  Cannot reach https://github.com/dotnet/arcade/tree/master/src/Microsoft.DotNet.Arcade.Sdk/tools/SourceBuild Status: 404
  
  Raw Output:
  message:"Cannot reach https://github.com/dotnet/arcade/tree/master/src/Microsoft.DotNet.Arcade.Sdk/tools/SourceBuild Status: 404" location:{path:"Documentation/planning/arcade-powered-source-build/README.md" range:{start:{line:121 column:3} end:{line:122 column:149}}} severity:ERROR source:{name:"linkspector" url:"https://github.com/UmbrellaDocs/linkspector"}
  Error: Process completed with exit code 1.