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.