No change in functionality, just some clean up.

In particular:

• move the code for running Sphinx to src/sage_setup/docbuild
• do not create and delete empty directories, instead silence the Sphinx warnings about non-existing directories
• instead of compile(), use import
• add a new variable SAGE_DOC_OUTPUT for src/doc/output (this is moved to local/share/doc/sage in Build documentation in $SAGE_SHARE/doc/sage #19963)

Depends on #19950

CC: @hivert @nathanncohen

Component: build

Author: Jeroen Demeyer

Branch: 0455d14

Reviewer: Marc Mezzarobba

Issue created by migration from https://trac.sagemath.org/ticket/19127