Move indexsidebar.html and switchers.js to docsbuild-scripts #90

The idea is that the cpython Doc/ repository should just care about building the documentation, not linking to other builds and languages from docs.python.org.

Building multiple versions, multiple languages, and linking them together is the role of docsbuild-scripts.

This would also avoid the need to update switchers.js and indexsidebar.html on every cpython branch for every new cpython release and every new language.

What would need to change:

• Drop switchers=1 from cpython/Doc/Makefile
• Drop if switchers block in cpython/tools/templates/layout.html. it already has an alternative. Add an id to the <li> so it can be easily found by the new switchers.js as the placeholders are dropped.
• Move switchers.js to docsbuild-scripts and make it use the <li> new id instead of placeholders if they are not found, and maybe fallback to find the place from other ways so it can work on old branches without commiting to them.
• Write a generic "indexsidebar.html" for local builds, to be commited in cpython, and move the "cross-linked" version of indexsidebar in docsbuild-scripts.
• Move {% trans %} tags in dummy.html so they can still be translated even if versionned in docsbuild-scripts (it's the only remaining part in cpython, hope it won't change too often).

We then need a clean way for docsbuild-scripts to edit the HTML to make it load switchers.js.