Add docs compile task to PR testing

As the doc_update task pushes updated .html files to source.wiredtiger.com it is prevented from running in patch builds, we don't want to update html files with incorrect or incomplete information.

However this also prevents developers from compiling their documentation changes to see if they compile in the evergreen environment.

We should split doc_update into two tasks:

• doc_compile, runs the compilation
• doc_update, pushes the files to the server

Doc_compile should be runnable in patch builds and should run in PR testing to detect issues prior to merge into develop.

#keepevergreengreen