Gather info about the wiki size

[ChiuchiuSorin]

A new Instance usage section should be added to include info about number of wikis, of documents, users composition from main wiki or subwikis.

This will use the Active Installs 2 API, so other things could be included later.

[oanalavinia]

A mockup for this section has been done by our designer. Note that only the main numbers section will be done as part of this issue, and it might suffer changes