chore: ensure new-docs are up to date (#5994)

* chore: ensure new-docs are up to date

This adds a Travis check that if we re-generate the docs we get the
exact output (by checking if the Git tree is dirty).

We do the dirty check by using `git status --porcelain`, seeing how many
lines that outputs, and using that as the exit code (taking only the
first 255 lines to avoid invalid exit codes). `--porcelain` makes the
output be empty if the repo is not dirty in anyway which translates into
an exit code of 0.

We can't use `git diff-index --quiet HEAD` as it exits with 0 if there are
untracked files in the repo but we want that to cause a failure.

.travis.yml

@@ -67,6 +67,7 @@ jobs:
         - npm run test-install
         - npm run lint
         - npm run test-doclint
+        - npm run ensure-new-docs-up-to-date
 
     # Runs unit tests on Linux + Firefox
     - node_js: "10.19.0"