doc: clarify use of +datum option with +proj=latlon

[Backport 8.1] DOC: Add content for Development/Error handling page.

DOC: Add content for Development/Error handling page.

* 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The document Development/Threads was severely out of date, and was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

I added the --all arg to the command, it seems its now a requirement, not sure if this is meant to be like this

Update docs About page to pull in license text from COPYING.

Doc: mentions EPSG methods that corresponds to topocentric and ortho (fixes #2832)