Prepare diagnostics docs for docc generation

Update the diagostic docs so that they can be built with docc:
    * Add technologyroot (diagnostics.md) to aggregate all markdown files
    * Fixed warnings in a few diagnostics docs

This allows the creation of docc documentation with a single
invocation of docc. It will generate the landing page from
diagnostics.md and include all of the diagnostics as subarticles.

The subarticles are accessible via the name of the markdown file,
which is being used as the ID for these diagnostics.
E.g. existential-any.

The docc docs for these documents can be generated locally with the
following command:
>docc preview --allow-arbitrary-catalog-directories userdocs/diagnostics

Author: daveyc123

Diff for: userdocs/diagnostics/.gitignore

@@ -0,0 +1 @@
+.docc-build

Diff for: userdocs/diagnostics/async-caller-execution.md

@@ -1,14 +1,15 @@
 # `AsyncCallerExecution`
 
-Proposed in [SE-0461], this feature changes the behavior of nonisolated async
+This feature changes the behavior of nonisolated async
 functions to run on the actor to which the caller is isolated (if any) by 
 default, and provides an explicit way to express the execution semantics for
-these functions:
+these functions.
+
+This feature was proposed in [SE-0461](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0461-async-function-isolation.md)
+
 * The `@execution(concurrent)` attribute specifies that a function must always 
   switch off of an actor to run.
   This is the default behavior without `AsyncCallerExecution`.
 * The `@execution(caller)` attribute specifies that a function must always 
   run on the caller's actor.
   This is the default behavior with `AsyncCallerExecution`.
-
-[SE-0461]: https://github.com/swiftlang/swift-evolution/blob/main/proposals/0461-async-function-isolation.md

Diff for: userdocs/diagnostics/diagnostics.md

@@ -0,0 +1,7 @@
+# Swift Compiler Diagnostics
+
+@Metadata {
+   @TechnologyRoot
+}
+
+Documentation on diagnostics emitted by the compiler.

Diff for: userdocs/diagnostics/existential-any.md

@@ -1,7 +1,9 @@
 # `ExistentialAny`
 
 This diagnostic group includes errors and warnings pertaining to the `any` type
-syntax proposed in [SE-0335].
+syntax.
+
+This syntax was proposed in [SE-0335](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0335-existential-any.md).
 `any` syntax draws a line between constraint types and existential or boxed
 types.
 
@@ -20,5 +22,3 @@ func sillyFunction(collection: Collection) { // error
   // ...
 }
 ```
-
-[SE-0335]: https://github.com/swiftlang/swift-evolution/blob/main/proposals/0335-existential-any.md