[DOCS] Add Keplr Android App

[DennisHartrampf]

Right now the docs say you need to install the Keplr browser extension. But now there's also an Android app that works fine.
Also the docs instruct to create a new wallet in Keplr. But it is also possible to import an existing wallet into Keplr and just add the Nillion testnet to the imported wallet.

[7-falseparfait]

is this stilll available ??