Support for in-place modification of docs

Adds cmd/mungedocs which is framework for processing
all files under docs/ and either verifying that no changes needed or
making in-place changes.

Did not reuse kube::util::gen-docs because that seemed to be
centered around handling added files, and this pass does not
add files.

Planned uses:
 - table of contents automatic updating
 - linkification
 - internal link checker
 - link-path-relativizer or absolutizer
 - example file syncer
 - header inserter.

Just table-of-contents updating in this PR.

Added Table of Contents to docs/networking.md.

Demonstrates use of new TOC generator presubmit.
Other docs will be added in future PRs.
Additional development will be needed to handle some
of the more complex cases.

hack/lib/golang.sh

@@ -64,6 +64,7 @@ kube::golang::test_targets() {
     cmd/integration
     cmd/gendocs
     cmd/genman
+    cmd/mungedocs
     cmd/genbashcomp
     cmd/genconversion
     cmd/gendeepcopy