Issue #157: #517
PR #568: #568

Co-authored-by: Oleh Prypin <oleh@pryp.in>

An extension here is simply a Python package
that defines an entry-point for a specific handler.

For example, an extension can add templates to the Python handler
thanks to this entry-point:

```toml
[project.entry-points."mkdocstrings.python.templates"]
extension-name = "extension_package:get_templates_path"
```

This entry-point assumes that the extension provides
a `get_templates_path` function directly under
the `extension_package` package. This function doesn't
accept any argument and returns the path to a directory
containing templates. The directory must contain one
subfolder for each supported theme, for example:

```
templates/
    material/
    readthedocs/
    mkdocs/
```

mkdocstrings will add the folders corresponding
to the user-selected theme, and to the handler's defined
fallback theme, as usual, to the Jinja loader.

The names of the extension templates must not
overlap with the handler's original templates.

The extension is then responsible, in collaboration
with its target handler, for mutating the collected
data in order to instruct the handler to use one of
the extension template when rendering particular objects.

For example, the Python handler will look for a `template`
attribute on objects, and use it to render the object.
This `template` attribute will be set by Griffe extensions
(Griffe is the tool used by the Python handler to collect data).

PR #569: #569