Rename this package (?), clarify what's in scope, and describe how to configure

## Naming

Is the focus of this repo really around "Docker images for Helia"?

From my poking, it seems like the real value of this repo is that it implements some of the HTTP Gateway APIs and uses Helia behind the scenes. One of the ways we package/distribute this is via Docker.

As a result, maybe call this helia-http-gateway ?

(This is similar to https://github.com/ipfs/bifrost-gateway. It has Docker distribution, but the defining attribute is that it provides "a lightweight IPFS Gateway daemon backed by a remote data store."

## Scope
1. Can we denote how much of https://specs.ipfs.tech/http-gateways/ we're attempting to implement currently?
2. Lets also call out what of https://docs.ipfs.tech/reference/kubo/rpc/ we're implementing
3. (I thought I saw it mentioned somewhere, but can't find it now.). Can we hook up the HTTP Gateway Conformance test suite to this? That is the canonical way to show how much this package supports. (Lets create a tracking issue for this if we don't have one.)

## Configurability
How does someone customize the Helia config that is used? I have no expertise on how much configurability should be allowed outside of the docker image build vs. asking users to create their own docker image. That said, we're definitely going to want configurability here (e.g., run this with "trustless gateway fallback" enable and disabled.)
```[tasklist]
### Tasks
- [x] Rename to helia-http-gateway
- [ ] https://github.com/ipfs/helia-http-gateway/issues/55
- [ ] https://github.com/ipfs/helia-docker/issues/19
```