graphile config doc overhaul

[jcgsville]

Description

Taking a full pass on the Graphile Config docs. Some highlights:

• Changing the 1-liner to "Graphile Config helps Node.js library authors make their libraries configurable
  and extensible", which I believe better conveys what the library aims to achieve when compared to "provides a standard plugin interface and helpers that can be used across the entire of the Graphile suite"
• Add a lot more detail around middleware & authoring plugins.
• Reorganize docs around needs of different users: library consumers, plugin authors, and library authors (though I haven't changed much of the content for library authors)

Checklist

• My code matches the project's code style and yarn lint:fix passes.

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[benjie]

This is great! Thanks so much for your work on this! 🙌

I've done a lot of editorial, but it's mostly just small edits for for clarity and correctness, or to align with my general writing tone.

I think we should reconsider restoring the middleware page though - plugins aren't just middleware (e.g. Graphile-Build plugins also have inflection, behaviors, gather, schema hooks, etc etc), middleware is just one of the standardized features that plugins offer. In future, I would like to add more dedicated pages for other features (e.g. if we standardize inflection across all our libraries) rather than making the plugin page longer...

Actually what would probably make sense would be to move plugin.md to plugins/index.md and then create plugins/middleware.md as well - group them together under the "plugin" banner, but still a dedicated page.

[jcgsville]

Thanks for the thoughtful feedback

[jcgsville]

This should be ready unless you want to do something about the ResolvePreset pseudocode

#2282 (comment)

[benjie]

Awesome; thanks for your hard work on this! 🙌