Use external test data

Remove Doozer CI

fix #1982:json_pointer.contains() exception is incorrectly raised

Merge branches 'develop' and 'external_test_data' of https://github.com/nlohmann/json into external_test_data

Fix README.md. Missing ```

About pull request:
https://github.com/nlohmann/json/pull/2074

An error has been introduced by accepting the suggestions:
https://github.com/ArthurSonzogni/json/commit/4be4a038ccd17aab0a4cc85200a6a0b41e1cccf2

One was about removing ~~~, but it was meant to be replaced by ``` in
reality. This caused the README.md to be slightly broken.

Fix error message about invalid surrogate pairs

Add CMake fetchcontent documentation and tests

I tested following strings with invalid surrogate pair and unpaired surrogate in files:

1. `"a\uD800\uD800x"`
2. `"a\uD800x"`

The error messge was: "... invalid string: surrogate U+DC00..U+DFFF must be followed by U+DC00..U+DFFF; ..."

I think it must be: "... invalid string: surrogate U+D800..U+DBFF must be followed by U+DC00..U+DFFF; ..."