doc: document how we use Travis CI

[fhinkel]

Our Onboarding and Collaborator guide only explain how we use Jenkins.

It's hard to tell if Travis CI is testing is redundant to Jenkins or not. I believe Travis CI builds the docs? Is green Travis CI required or optional for landing PRs?

Does anybody want to take this on and add one or two sentences to our onboarding docs? Thanks!

[richardlau]

At the moment Travis is advisory and completely optional (none of our tooling looks at the results). This may change (#29770). (As an aside I'm hesitant to switch over main build and test to Travis as I can't guarantee we'll be able to stay under Travis' current 50min build time limit (we're right up to it if large parts of V8 need recompilation).)

The only thing Travis currently tests that isn't tested on Jenkins is the commit message linting and that is done as part of node-core-utils if you are using it to land PRs.

[richardlau]

Mentioning Travis is one of the checkboxes in #30116.

[fhinkel]

OK, thanks!