For a while now we've been gathering some codebase-level docs that felt
out-of-place on the main Terraform website (since they are about the
implementation, not usage) but we had no existing suitable place to put
them.
In order to make this information more available (and, hopefully, more
likely to stay up-to-date as we change things), here we'll establish the
"docs" directory as a place to keep documentation aimed at those who are
working on code changes to the Terraform Core codebase.
User-oriented docs should never appear in this directory. The Terraform
website is always the better place for those. The set of docs here is
rudimentary to start and we'll see if it makes sense to expand and
reorganize it over time based on the experience with having these initial
docs available.
