Lack of Onboarding Documentation

**What is wrong with the docs?**

There seems to be a relative absence of onboarding documentation. Cabal is a complex project, with a SLOC via sloccount of about 132k lines of Haskell.

As far as I'm aware, the current Cabal team is relatively overworked, and moreover, there's been multiple failed or incomplete attempts (I think ptkato attempted it, VeryMilkyJoe has a partial attempt), to build an exact parser that, in my impression, ended up getting stonewalled by a lack of understanding of the existing parsing system.

I myself am still trying the project, but am budgeting at least 6 weeks (this may easily grow into 18 weeks, but you have to set a deadline) to understand the present Cabal project.

To make Cabal more accessible, to make it easier to onboard new helpers, putting out onboarding documentation will make it easier for people to contribute to what is a storied and prominent project in the Haskell ecosystem.

**Additional context**
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