Add main branch to output

[lcawl]

This PR updates the conf.yaml to build both "main" and "master", so that we can move away from the latter.

[lcawl]

Created elasticsearch-cn/elasticsearch-php#2 to address this build error:

08:59:56 INFO:build_docs:Bad cross-document links:
08:59:56 INFO:build_docs: /tmp/docsbuild/target_repo/html/cn/elasticsearch/php/6.0/_community_integrations.html contains broken links to:
08:59:56 INFO:build_docs: - en/elasticsearch/guide/master/index-aliases.html

[lcawl]

These changes no longer seem necessary due to our migration plan.