Doc target completions needs to take into account file locations

[alcarney]

Offering a completions for the :doc: role need a little more intelligence behind them. Consider the following hierarchy

docs
|- a
|   |- x.rst
|   \- y.rst
\- b
    \- z.rst

Typing :doc:`b/z` in a/x.rst will result in Sphinx not being able to resolve the reference.
It either needs to be

• a relative reference to the a/x.rst file (:doc:`../b/z` )
• an "absolute" reference, relative to the project's conf.py (:doc:`/b/z` )

The insert_text for the target's CompletionItem needs to be tweaked to fix this