Add description for Usage modal

[oanat]

Add a description for the usage modal Wiki size:

• On the store page https://store.xwiki.com/xwiki/bin/view/Extension/AdminToolsPro#documentation
• On the page View usage info for all wikis > Wikis size itself

We should also include:

• How is each column computed?
• Comparison to the total storage needed by the wiki to run (this would be useful in a cloud context).

[ChiuchiuSorin]

Add a description for the usage modal Wiki size:
On the store page https://store.xwiki.com/xwiki/bin/view/Extension/AdminToolsPro#documentation

I updated the documentation to include more info regarding the instance size and usage modal.

On the page View usage info for all wikis > Wikis size itself

The UI should be updated in #160 .

Comparison to the total storage needed by the wiki to run (this would be useful in a cloud context).

Cloud details were added in #77 and #76 . Currently, the testing of these features is depending on the release of the new version of XNNG.