[DOC] Improve SOCI documentation

[djdongjin]

Is your feature request related to a problem? Please describe.

The GETTING_START doc is too cumbersome (install dep, build/install soci, set up local registry, etc) and need some update.

We should provide individual docs instead of putting all in a GETTING_START, such as:

• [x] GETTING_START: this doc is for general users and go through SOCI from installation, index create/push/pull, to running container with lazy loading. It should minimize required setup (e.g., download binaries instead of build from source) and mimic real scenario usage (e.g., use public registry instead of local container registry).
• [x] BUILD (1 day): this doc is for developers and covers build-dependencies (with install scripts (https://github.com/awslabs/soci-snapshotter/blob/main/scripts/install-dep.sh) or commands), build SOCI and install binaries, how to run unittest/integration test.
• [x] INSTALL (1 day): this doc covers how to install SOCI for containerd with systemd.
• [x] DEBUG (2 day): this doc covers how to debug soci-snapshotter in general. This’ll help users/operators debug issues from SOCI.
• [ ] technical-snapshotter (2-3): this is the main technical doc covering different components of SOCI (e.g., SpanManager, BackgroundFetcher, etc) and how it interacts with containerd and remote registries. #756
• [x] technical-soci-index (2-3): this doc focuses on SOCI index including index/ztoc/toc/zinfo, and how they’re related to layers (and files in layers). #757
• [x] REGISTRY (2-3): this doc tracks SOCI support status (OCI 1.1, OCI 1.0, not support) on popular registries such as ECR, dockerhub, GHCR. So users can know if their registries are already supported before trying SOCI.
• [x] Example config toml with all config options and default values. #758
• [X] Make verbiage consistent across docs referring to soci project, snapshotter, binaries, etc. #709
• [x] Document the authorization process, options, etc. #736

Describe the solution you'd like

Better documentation, especially GETTING_STARTED and REGISTRY_SUPPORT (with support status on popular registries like dockerhub/ECR/ACR/GCR/GHCR).

Describe alternatives you've considered

No. We need better documentation.

Additional context

I think this is fairly high priority. soci is a open source project. If users cannot try soci easily following a straightforward doc, they probably won't come back.

[djdongjin]

@Kern-- I updated this one with a list of doc/subtasks and my estimation for individual docs (just for reference). This one is ready for estimation.

[Kern--]

@djdongjin to take BUILD/INSTALL @turan18 to take DEBUG @Kern-- to take REGISTRY

[sparr]

709 asks for consistent verbiage to refer to soci binaries, etc, being implemented in #730

[sparr]

736 asks for documenting the authorization process