The specific code in Makefile.in for the documentation and its dependencies is moved to the new script packages.

In preparation for #32759:

• We introduce another installation tree (in addition to SAGE_LOCAL and SAGE_VENV), called SAGE_DOCS. When the installation tree variable is empty, we suppress building the package.
• We introduce a mechanism to select an installation tree, the file trees.txt. Using this mechanism, we associate the new script packages with the installation tree SAGE_DOCS.

Next step:

• ./configure --disable-doc #32759 ./configure --disable-doc
• pip-installable packages sagemath-doc-src, sagemath-doc-inventory, sagemath-doc-html, sagemath-doc-pdf #29868 pip-installable packages sagemath-doc-src, sagemath-doc-inventory, sagemath-doc-html, sagemath-doc-pdf
• rename src/doc/bootstrap to build/pkgs/sagemath_doc_html/bootstrap.

Depends on #30383

CC: @jhpalmieri

Component: documentation

Author: Matthias Koeppe

Branch/Commit: fd298c4

Reviewer: John Palmieri

Issue created by migration from https://trac.sagemath.org/ticket/31356