[Website] On change to master branch, automatically make PR to asf-site

[asfimport]

I added a build/deploy script to arrow-site that would enable automatically publishing to asf-site when there is a commit to the master branch. However, ASF won't let us add a deploy key to enable this publishing (INFRA-18924).

I have a workaround that's not automatic but as close as we can get. On commits to apache/arrow-site's master branch, Travis builds the site and pushes it to a fork of arrow-site (where there is no restriction on deploy keys), and then it makes a PR from there back to the asf-site branch of apache/arrow-site using hub. So it's "semiautomatic": the asf-site PR is made automatically, but a committer will need to merge it.

Reporter: Neal Richardson / @nealrichardson Assignee: Neal Richardson / @nealrichardson

Related issues:

• [Website] Add support for auto deployment by GitHub Actions (is superceded by)

  PRs and other links:

• GitHub Pull Request #21

_{Note: This issue was originally created as ARROW-6497. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou: I changed wrong issue. ARROW-6569 should have been changed...