[Docs] Update the documentation contents and index (swiftlang#34157)

* Update docs/README.md
* Update docs/contents.rst

Author: benrimmington

Diff for: docs/README.md

@@ -113,7 +113,7 @@ documentation, please create a thread on the Swift forums under the
     Describes how the "Omit Needless Words" algorithm works,
     making imported names more idiomatic.
 - Type-checking and inference:
-  - [TypeChecker.rst](/docs/TypeChecker.rst):
+  - [TypeChecker.md](/docs/TypeChecker.md):
     Provides an overview of how type-checking and inference work.
   - [RequestEvaluator.md](/docs/RequestEvaluator.md):
     Describes the request evaluator architecture, which is used for

Diff for: docs/contents.rst

@@ -12,7 +12,6 @@ Contents
    Generics
    StoredAndComputedVariables
    SIL
-   TypeChecker
    OptimizationTips
    ABI: TypeMetadata <ABI/TypeMetadata>
    ABI: TypeLayout <ABI/TypeLayout>