drop dependencies from docs/main.js

[nmushegian]

• drop ethers completely and use dmap.js only, refactor dmap.js to use a provider directly, not an ethers object
• sha256
• • I think we should focus on making sure the endpoint can be configured rather than verifying CIDs -- your endpoint is supposed to be a local client you trust to do the hash check, in any case, utils.js currently imports entire multiformats
• (IPFS) node.cat
• • replace to a request to the endpoint, you can use fetch or whatever, it is a simple rpc