We deprecate the package sage.docs which contains two modules conf and instancedoc.

The module conf is not one of normal sage modules, but is the Sphinx configuration for documentation building. Hence we move the module into sage_docbuild.

The module instancedoc provides a decorator to add docstrings to instance objects. This has nothing to do with building documentation. Hence we move it to sage.misc.

This ticket also refactors the code in sage_docbuild.__init__. The "builders" are moved to a new module sage_docbuild.builders and the "main" function is moved into sage_docbuild.__main__.

Finally we add some documentation about Sage's documentation system.

To minimize confusion, this ticket should be merged with sage 9.7.beta as early as possible (before any other tickets touching conf.py)

CC: @antonio-rojas

Component: documentation

Author: Kwankyu Lee

Branch: d8a645f

Reviewer: Matthias Koeppe

Issue created by migration from https://trac.sagemath.org/ticket/33763