Upload intersphinx mapping to documentation homepage

[Philipp]

Upload the intersphinx mapping (objects.inv) to the documentation homepage so other projects can reference the lxml types

It is created under
./doc/html/apidoc/objects.inv
when ```make apihtml``` is run sucessfully

[Timo Brembeck (timo-42)]

The intersphinx mapping is already available at:

https://lxml.de/apidoc/objects.inv

so it can be referenced in the `conf.py` via:
```
intersphinx_mapping = {
    "lxml": ("https://lxml.de/apidoc/", None),
}
```