Separate PR template for website changes only

Currently we are using single template https://github.com/apache/beam/blob/master/.github/PULL_REQUEST_TEMPLATE.md for any new PR. However, those postcommit tables are not needed for website changes. And we would like to include staged link which is automatically created by Jenkins beam_PreCommit_Website_Stage_GCS_Commit (see details [here](https://cwiki.apache.org/confluence/display/BEAM/Website+Tips)). It would be much easier for people to review and find problem.

So we can use separate PR template to do it. The instruction of multiple template can be found in https://help.github.com/en/articles/creating-a-pull-request-template-for-your-repository

Imported from Jira [BEAM-7985](https://issues.apache.org/jira/browse/BEAM-7985). Original Jira may contain additional context.
Reported by: markflyhigh.