DOC: Refresh Development/Quickstart and remove Development/Threads (#2863)

* DOC: Refresh Development/Quickstart and remove Development/Threads

The document Development/Threads was severely out of date, and was
focussed on contrasting the current API with an old API that was
deprecated several years ago, and removed from the current PROJ earlier
this year.

Update Development/Quickstart to give more information about the use of
threading context, as a replacement for the content that was previously
in Threads.

Also give the example code examples/pj_obs_api_mini_demo.c a cleanup
pass to make it at least internally consistent with its own code style.

The header comments of the example code are still, much like the Threads
document, fixated on comparing proj.h against proj_api.h, which is an
interesting historical curio to be sure, but probably doesn't need to
persist here.  It might be worth cleaning this up further as a separate
exercise.

Fixes #2451

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