Documentation/fix build docs

[vadimreutskiy]

Description of the Change

Changed a few small things in the documentation:

• Added note about no need to compile Iroha to start using it.
• Fixed list format at the "Building" documentation page
• Added example for key generation procedure

Benefits

Users will struggle less with their frequent questions.

Possible Drawbacks

Not in this case

[neewy]

Just a hint for the future: if you update the docs include [ci skip] to your commits so that CI is not triggered for your build