Tristan/architecture docs

See merge request !907

This commit introduces BST_GRANULAR_STAGE as a compatibility flag,
so Elements can assert that calling stage_dependency_artifacts,
integrate_dependencies and post_integration_staging
is equivalent to calling stage.

This will be needed to implement staging multiple elements,
since the integration commands for all elements need to be called
after all dependencies are staged.

This makes it prefer to call stage_dependency_artifacts,
integrate_dependencies and post_integration_staging in-turn
if BST_GRANULAR_STAGE is set,
so the implementation of Element.stage may be omitted.

The prepare_sandbox implementation looks a little convoluted
because for Scope.BUILD and self.BST_GRANULAR_STAGE
we prefer to run the granular methods,
but we also run most of them for Scope.RUN
except the extra parts of stage() that were post-integration.

This makes the __layout check per-method
and makes ScriptElement.stage call the methods in turn.

stage_dependency_artifacts falls-back to the default implementation
for everything except Scope.BUILD since Element.stage was only called
for Scope.BUILD, so layout handling shouldn't be consulted for other scopes
for compatibility with existing behaviour.

This is mostly just marking which elements work as expected
from changes to their base class.

Junction and Filter elements expect overriding stage to be sufficient
to prevent it doing anything in those contexts,
and since Element.stage is going to be hanging around
there's no pressing need to port everything.

There's redundancy in pre-staging sandbox creation logic
with Element._sandbox doing something similar,
but hopefully not too much.