drop dependencies from docs/main.js

- 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

Read docs/main.js, dmap.js, and utils.js first to map the current dependency and endpoint flow. Check how the endpoint is configured and how node.cat, hashing, and multiformats are currently used. Done means the requested endpoint-based flow works without ethers and the resulting behavior is covered by the project's available checks.

Written by the indexing model from the issue text.