This commit adds a conf.py and index.rst to src/doc/en which
allow for building the whole of Sage English documentation
as a single info manual.

The conf.py is almost the same as the one in
src/doc/en/reference except it doesn't exclude the subdirs.

To build the info manual,

```
cd src/doc/
make doc-src
cd -
./sage --docbuild reference inventory
./sage -sh -c "sphinx-build -M info ./src/doc/en target"
makeinfo --no-split --force -o target/sage.info target/sage.texi
```

Some files have paths that are like ../sage/... but ../sage
doesn't exist and referred to files are actually in sage