Fix dead links

Author: drewnoakes

doc/automation/obtaining_the_MSBuild.Project_from_CPS.md

@@ -7,7 +7,6 @@ Obtaining the MSBuild.Project from CPS
 3. Acquire a read, upgradeable read or write lock, as appropriate, and 
    use the MSBuild Project object exclusively within the lock:
 
-Visual Studio 2019
 ```csharp
 await projectLockService.WriteLockAsync(
     async access =>
@@ -23,24 +22,10 @@ await projectLockService.WriteLockAsync(
     cancellationToken);
 ```
 
-Visual Studio 2017 and earlier
-```csharp
-        using (var access = await projectLockService.WriteLockAsync())
-        {
-            MSBuild.Project project = await access.GetProjectAsync(configuredProject);
-
-            // Use the msbuild project, respecting the type of lock acquired.
-
-            // If you're going to change the project in any way, 
-            // check it out from SCC first:
-            await access.CheckoutAsync(configuredProject.UnconfiguredProject.FullPath);
-        }
-```
-
 Note that it's important that you use `await`. Do not use `Task.Result` or
 `Task.Wait()` on these async methods or your code will malfunction and/or hang.
 If you must do this within a synchronous method, see [threading 
-rule #2](https://github.com/Microsoft/vs-threading/blob/master/doc/threading_rules.md#2-when-an-implementation-of-an-already-shipped-public-api-must-call).
+rule #2](https://github.com/microsoft/vs-threading/blob/main/docfx/docs/threading_rules.md#-rule-2-use-jtfrun).
 
 **Please observe CPS [project locking rules](../overview/project_lock.md) by not
 retaining any references to MSBuild objects beyond the scope of the lock and

doc/overview/mef.md

@@ -228,7 +228,7 @@ IMyConfiguredComponent c2 = configuredProject.Services.ExportProvider.GetExporte
 ```
 
 For details on obtaining an instance of `UnconfiguredProject` or `ConfiguredProject`,
-see [Finding CPS in a VS project](..\automation\finding_CPS_in_a_VS_project.md)
+see [Finding CPS in a VS project](../automation/finding_CPS_in_a_VS_project.md)
 
 ## MEF and C# Nullable Reference Types
 

doc/overview/threading_model.md

@@ -6,7 +6,7 @@ Rules](threading_rules.md)
 Important: 
 
 **Do not call Task.Wait() or Task.Result in your code because these
-violate [threading rule #2](https://github.com/Microsoft/vs-threading/blob/master/doc/threading_rules.md#2-when-an-implementation-of-an-already-shipped-public-api-must-call) and will often lead
+violate [threading rule #2](https://github.com/microsoft/vs-threading/blob/main/docfx/docs/threading_rules.md#-rule-2-use-jtfrun) and will often lead
 to deadlocks.**
 
 Avoid use of `ThreadHelper.JoinableTaskFactory` as that does not have the intelligence