docs: separate template syntax into multiple docs (#36954)

This is part of a re-factor of template syntax and
structure. The first phase breaks out template syntax
into multiple documents. The second phase will be
a rewrite of each doc.

Specifically, this PR does the following:

- Breaks sections of the current template syntax document each into their own page.
- Corrects the links to and from these new pages.
- Adds template syntax subsection to the left side NAV which contains all the new pages.
- Adds the new files to pullapprove.

PR Close #36954

aio/content/guide/aot-compiler.md

@@ -623,7 +623,7 @@ For more information about input type narrowing, see [Input setter coercion](gui
 
 ### Non-null type assertion operator
 
-Use the [non-null type assertion operator](guide/template-syntax#non-null-assertion-operator) to suppress the `Object is possibly 'undefined'` error when it is inconvenient to use `*ngIf` or when some constraint in the component ensures that the expression is always non-null when the binding expression is interpolated.
+Use the [non-null type assertion operator](guide/template-expression-operators#non-null-assertion-operator) to suppress the `Object is possibly 'undefined'` error when it is inconvenient to use `*ngIf` or when some constraint in the component ensures that the expression is always non-null when the binding expression is interpolated.
 
 In the following example, the `person` and `address` properties are always set together, implying that `address` is always non-null if `person` is non-null.
 There is no convenient way to describe this constraint to TypeScript and the template compiler, but the error is suppressed in the example by using `address!.street`.