This will be the place to store low level data types used
throughout the core, for now this includes public and private
types.

And slightly touch up the doc strings for Consistency and Scope.

Refactor types api

See merge request !870

* Add a link to the website on the main page.
* Remove install instructions as they are now on the website.
* Remove Resources section as that information can be found at the
  website, and also looks bad.
* Move artifact server setup from the no longer existing Install section
  to the Using section.

The docker guide, which is part of the install guide, has moved
to the website.

Remove install guide

See merge request !872

We shouldn't need it to persist now that we cache failed build dirs.

This change breaks the test
`tests/integration/shell.py::test_sysroot_workspace_visible`.
I can no longer see a use-case for this test.
AIUI, it tested that the failed build sysroot stored in the builddir has
the workspace's files in, despite the workspace being unmounted.
I believe this behaviour is made redundant by cached buildtrees.

This fixes part of #539

This includes changes in app.py:
* Interactive failure shell no longer uses the failed build sysroot,
  defaulting to the cached build tree.
* Always allow producing a failure shell.

Changes in element.py are:
* Errors caused by building don't store the failed build sysroot.
* When staging sources, will stage the element's cached build tree if it
  exists.

This fixes part of #539

This is related to #539

i.e. instead of taking a directory that must contain "root" and
"scratch", and treating "root" as the root, use the directory directly.

In element.py:
* __sandbox takes the `bare_sandbox` arg, to pass into the sandbox's constructor

In sandbox.py:
* If bare_sandbox, `_root` is the passed-in directory, and `__scratch`
  is None.
* Trying to use `__scratch` when bare_sandbox is True is a bug.

In _mount.py:
* Don't get the value of `__scratch` if it's not needed.

This is part of #539

This is related to #539