Move and expand contributing.md content into docs site

[micahellison]

Documentation Change

There is a lot going in our contributing.md file and we're thinking of adding more. Moreover, we have several different audiences for this file (docs writers, testers, and developers), and we might be able to reach more of them through the jrnl.sh site.

It seems like it would be a good move to create a new section in the documentation for contributors, with subsections for each of these audiences.

As part of this, the developer documentation could be expanded to include some information on the project's architecture and our process for pinning and resolving GitHub issues.

After this work is done, the contributing.md shouldn't be removed, but it should mainly just link to this new documentation.

[stale]

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