Add some advocacy information

@n1k0 collected the following feedback from Sudweb:

```
the person told me that once you're "decided" to learn about kinto it does the job
he conceded that the main issue was probably to actually make peoiple wanting to learn about it in the first place :)
```

Maybe we can add some more advocacy information: Kinto is great at X, Y, and Z, or you should use it because A, B, and C.

Start by reviewing the current Kinto website content and the feedback quoted in this issue. Determine which advocacy information is missing and agree on what Kinto should promote before making changes; the work is done when the website clearly explains why people should use Kinto.

Written by the indexing model from the issue text.