Replace Device Authentication Overview diagram and move to assets folder

[szczys]

This diagram is blurry: https://github.com/golioth/docs/blob/b2aa810d9ffa255c16803cf8784327962c331ee2/docs/firmware/firmware-client-auth.drawio.svg

Compare this diagram to the one in the LightDB Client overview:

• https://docs.golioth.io/firmware/device-sdk/authentication/overview-auth
• https://docs.golioth.io/firmware/device-sdk/light-db/overview-lightdb

We should fix the appearance and move the file to the assets folder.

[szczys]

This was fixed with #258