refactor: Deprecate 'selection' and 'rendering' YAML keys

[pawamoy]

Commit message:

As seen in commit eb822cb, the separation of
the selection/collection and rendering configuration
is problematic. This commit further reduces this
separation by merging the two current YAML keys
'selection' and 'rendering' into a single
'options' key. It means both the 'collect' and
'render' methods have access to every option.

The old keys can still be used, but are deprecated,
and therefore a deprecation warning is emitted
if these keys are found in either global or local
configurations.

---

This was mentioned in #364.
Example diffs:

 # mkdocs.yml
 plugins:
 - mkdocstrings:
     handlers:
       python:
-        selection:
+        options:
           docstring_style: google
           docstring_options:
             ignore_init_summary: yes
-        rendering:
           merge_init_into_class: yes
           show_submodules: no

 # My Class

 ::: my_package.MyClass
-    rendering:
+    options:
       show_root_toc_entry: no

[pawamoy]

I intend to merge this in two days 🙂

[pawamoy]

Merging, we can always revise the changes before releasing them.