Expand documentation on contents of repository

[sjspielman]

I'm opening this issue based on a couple docs-related thoughts I had while reviewing #284. That PR was the first time I had opened/thought about build-index.nf, as a newer contributor to scpca-nf. Before looking at any code, I went to look for specific docs about how this workflow differs from the main workflow, but couldn't find any. When I went to look at the code, I got a sense of what the workflow does on my own (or did I?), but it got me thinking about some areas we can expand docs in this repo:

• A Workflows in this repository section in the main README.md (or elsewhere?) would be helpful. I envision some very quick bullet points introducing the (now 3!) different nextflow files (and maybe also nextflow.config?) this repo holds.
• Along those lines, currently there do not appear to be any docs about build-index.nf. It looks like in the past (Start addition of instructions for external users to README #106) there was a decision (or the discussion didn't really get wrapped up?) to not include this in external-instructions.md, but it really should be documented somewhere. Documenting it via the first bullet point about (workflows in this repo docs) could get the job done, but I think some info in internal-instructions.md (perhaps CONTRIBUTING.md?) would be helpful.
• A README.md in the config/ directory giving a quick explanation for what type of information is expected in each config. How do their intended roles differ? Some of this is cooked into their names, but some more formal docs on it would be helpful too. This comment is what made me think such docs are important: Add process for training celldex references for SingleR #284 (comment)
• A README.md in the bin/ directory could indicate which workflow(s) use which script(s). Right now this would not be terribly interesting, but as we build up the integration and celltyping code, I imagine this will become increasingly useful.

[sjspielman]

Noting some aspects of this got added as part of #500