Improve logging

[humitos]

There are some issues around logging that can be improved,

1. Warning seems to be "too aggressive" in the Sphinx content. People uses a lot -W when building their docs on CI systems. This means that their builds fails because hoverxref logs some warnings.
2. Warning on hoverxref not fully configurated, may be better as INFO as well. Otherwise, people need to force the definition of it in their conf.py --when the extension only works on Read the Docs and this is automatically configured.
3. A lot of log.info is thrown in the output mentioning the data that was injected. This is useful when debugging only, so it may be good to change to that level.
4. Usage of default role it's better candidate for an info (at this line). We already have a config that sets the default role.

I'm not super happy introducing these warning changes in the latest release because it broke some users builds. However, they seem to be good warnings by default, so the first time you install the extension you are able to configure it completely.

References: scrapy/scrapy#4480

[humitos]

Done at #64