Update "Contribution Process" page at Confluence

https://cwiki.apache.org/confluence/display/KNOX/Contribution+Process#ContributionProcess-3.3.TravisCI needs few small updates:

1) The url (https://travis-ci.org/apache/knox) is outdated. It should be https://travis-ci.*com/github*/apache/knox

2) "Pull requests should not be merged if the Jenkins job fails to validate the merge." - this should be TravisCI

3) It could mention that the PRs are tested on both AMD64 and ARM64 (see https://issues.apache.org/jira/browse/KNOX-2535)

4) Maybe a new item could be added about the usage of GitHub Actions (https://github.com/apache/knox/blob/master/.github/workflows/main.yml)