Add .md extension to internal links (#4214)

docs/help/contributor/github.md

@@ -188,7 +188,7 @@ the known commands:
 - You can also use Prow to manage labels on PRs with `/kind ...`,
   `/good-first-issue`, or `/area ...`
 
-- See the [Branches](structure/#branches) section for details about how
+- See [Branches](structure.md#branches) for details about how
   to use the `/cherrypick` command.
 
 ### Common GitHub PRs FAQs

docs/help/contributor/structure.md

@@ -12,7 +12,7 @@ blog post, documentation within a code repo, or a vendor site may be the right
 place. Be generous with offering to link to such locations; documents in the
 main documentation come with an ongoing cost of keeping up to date.
 
-[Learn about documenting code samples](/code-samples.md)
+[Learn about documenting code samples](code-samples.md)
 
 
 ## Branches

docs/help/contributor/templates/template-procedure.md

@@ -71,7 +71,7 @@ For example:
 
 Before you create the `helloworld-java` sample app, you must have a Kubernetes cluster with Knative
 installed and DNS configured.
-See the [installation instructions](../../../install/) if you need to create one.
+See the [installation instructions](../../../install/README.md) if you need to create one.
 
 
 ## Task 1