Improve docs markdown generation, allow example and extended

[carlopi]

Add possibility for extension to have two extra fields in the descriptor file:

• docs.hello_world: an optional SQL based example of what capabilities the extension provides
• docs.extension_description: allow to pass an optional extra paragraph (markdown enabled) to describe / provide additional context on the extension

That allows to generate markdown files, that once passed down to the website generation outputs something like:

HTML side improvements have to be made, but those are orthogonal on deciding what content should be there.