[docs] Tweak the "Edit this page" button positioning

[danilo-leal]

• [x] I have followed (at least) the PR section of the contributing guide.

This PR makes the Edit this page button aligned to the left (flex-start) instead of to the right. It just occurred to me that this layout is more balanced as the button is hanging alone by the right. Additionally, I switched the icon from the edit pen to the GitHub logo so it's clear for readers they'd be redirected to the page's code (getting inspiration from Ionic's docs in this aspect).