Test for OTP matrix

[paulo-ferraz-oliveira]

This will, e.g., allow us to catch OTP 27 (at the moment) errors early on in the release cycle.

~I'm in no hurry, and this can most likely wait until 27 is actually out, after which we'd adjust the target OTP.~

This:

• adds a rebar3-based library (for testing, in examples)
• runs rebar3 ex_doc on that library (generated/saved in an Elixir env., but recovered/restored outside of it)
• uses _checkouts after we generate priv to make sure we're on top of the latest rebar3_ex_doc code

Reference

This was also discussed in:

• https://erlangforums.com/t/erlang-otp-27-0-rc1-released/3300