[ECO-2115] Make assorted updates to indexer framework

[alnoki]

Description

1. Update README for new submodule clone style
2. Use image-specific tagging for enhanced granularity in image development/dependency management per https://github.com/econia-labs/emojicoin-dot-fun/pull/199#discussion_r1727773203
3. Defer branch-based image generation until development process has stabilized
4. Clarify that processor is its own workspace
5. Remove broker gitignore and lockfile, which are both not needed due to the refactored package layout
6. Move broker README out of Rust src folder
7. Use port env vars in compose port mappings
8. Update example.env for testnet values and new env vars
9. Remove unused CONTRACT_ADDRESS env var from processor
10. Add README section for running Docker compose
11. Update indexer README header hierarchy for new directory layout
12. Use linux/amd64 image builds instead of cross-platform to accelerate builds for inclusion with CloudFormation, which uses amd64

Testing

• [x] Successfully ran Docker compose locally
• [x] Push broker to Docker hub: https://hub.docker.com/repository/docker/econialabs/emojicoin-dot-fun-indexer-broker/general
• [x] Push processor to Docker hub: https://hub.docker.com/repository/docker/econialabs/emojicoin-dot-fun-indexer-processor/general

Checklist

• [x] Did you update relevant documentation?
• [x] Did you check all checkboxes from the linked Linear task?

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Name	Status	Preview	Comments	Updated (UTC)
emojicoin-dot-fun	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Aug 22, 2024 11:28pm
emojicoin-dot-fun-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Aug 22, 2024 11:28pm