Scope: Apply a consistent pattern for #include. Note in the developer documentation.

Proposal: use #include <mongoc/...> and #include <bson/...> instead of #include "..." within the source. Apply to internal files too.

Rationale: Consistent use of <> ensures include directories are being specified correctly via the build system, rather than bypassing valid include directory specifications through relative path lookup.

Background & Motivation:

There are different include patterns in the C driver. Example:

#include "mongoc-prelude.h" // In src/libmongoc/src/mongoc/mongoc-array-private.h
#include <mongoc-prelude.h> // In src/libmongoc/src/mongoc/uthash.h

As well as whether or not to prefix with mongoc or bson:

#include <mongoc/mongoc-error.h> // In src/libmongoc/src/mongoc/mcd-nsinfo.c
#include "mongoc-error.h" // In src/libmongoc/src/mongoc/mongoc-topology.c

This has caused difficulties for downstream users building the driver outside of CMake. Example: https://github.com/mongodb/mongo-c-driver/pull/1836