Adding graviton logo and badges

[aorumbayev]

Proposed changes

• Adds 2 variations on a proposal for graviton logo inspired by this diagram. Main readme contains colored gradient background (teal and green from algorand's website), and tutorial page is reversed black background and gradient on text itself.
• I noticed that markdownlint is used so i had to add one extra disable for a rule that would prevent having header image as a first content in md file. If you'd like to follow the convention though - feel free to to suggest moving text above the icon.
• Images are hosted on ipfs and here is a .psd version also on ipfs -> ipfs://QmVcS8ULiEUrzs3m6bFQncuRt4MUV8ydnJmyen2wWCt5uq, should be possible to download it and edit in photoshop further if needed.
• Lastly adds 2 tiny repo badges (visitors count and link to algorand website), could be used as a starter template in case you wanna add more badges (like coverage, ci status and etc) in future.

[CLAassistant]

All committers have signed the CLA.

[tzaffi]

Thank you for this great addition!!!!

[aorumbayev]

@tzaffi btw this is offtopic, i've noticed you guys manually install the docker-compose in the default ubuntu 20 github actions worker. I suppose there is a need on your side to rely on particular docker compose version or its somehow tied to that sandbox custom github action. But if not, check out the list of tools that are already pre-installed into github actions workers (specifically ubuntu-latest), docker and docker-compose are already installed on these workers, so you can potentially simplify the ci file a bit -> https://github.com/actions/virtual-environments/blob/main/images/linux/Ubuntu2004-Readme.md#tools

[tzaffi]

@tzaffi btw this is offtopic, i've noticed you guys manually install the docker-compose in the default ubuntu 20 github actions worker. I suppose there is a need on your side to rely on particular docker compose version or its somehow tied to that sandbox custom github action. But if not, check out the list of tools that are already pre-installed into github actions workers (specifically ubuntu-latest), docker and docker-compose are already installed on these workers, so you can potentially simplify the ci file a bit -> https://github.com/actions/virtual-environments/blob/main/images/linux/Ubuntu2004-Readme.md#tools

Thanks. I've created #29 as a follow up.

[aorumbayev]

@tzaffi awesome, thanks for offloading this to a gh issue!

But otherwise, will await for you to merge, unless there are any extra comments for the pr 👍