Add doxygen entries for:

- proj_context_set_file_finder
- proj_context_set_ca_bundle_path

Fixes #2540

Refs #2540.

Fixes #2512

Fixes #2779

proj_factors(): accept P to be a projected CRS (fixes #2854)

Updated doc:

    Starting with PROJ 8.2, the P object can be a projected CRS, for example
    instantiated from a EPSG CRS code. The factors computed will be those of the
    map projection implied by the transformation from the base geographic CRS of
    the projected CRS to the projected CRS.

    The input geodetic coordinate lp should be such that lp.lam is the longitude
    in radian, and lp.phi the latitude in radian (thus independently of the
    definition of the base CRS, if P is a projected CRS).

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The document Development/Threads was severely out of date, and was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

This page was previously empty.  This commits populates it with some
very basic information and a code example of how to use
proj_context_errno and proj_context_errno_string.

Something: because something is better than nothing.

It might be good to include some more fine detail about how PROJ manages
the error state internally, but I don't yet have the expertise in PROJ
internals to write that content myself.

Also the code sample probably has a segfault bug or leaks memory,
because C.