Linter to enforce docs style guide

[frankfarzan]

We're going to automatically enforce a style guide on all documentation (reference, book, catalog). This will be a growing list of rules, starting with:

• [x] All references to the tool should be kpt (small case).
• [x] No broken links (I believed this is enforced already)
• [ ] Banned words: hydration -> render
• [ ] Function images must be versioned, disallow unstable tag
• [ ] All verbatim shell quote blocks should start with $. Right: $ kpt pkg get Wrong: kpt pkg get
• [ ] Links to other kpt.dev pages should be relative
• [ ] Detect TODO(s)