Render examples from newlib_tests into docs

[pevik]

kerneldoc in struct tst_test is quite long already but it does not contain examples.

It'd be nice to modify https://github.com/linux-test-project/ltp/tree/master/doc/conf.py to cat content of (some) tests in https://github.com/linux-test-project/ltp/tree/master/lib/newlib_tests/ either into example section (not yet created).

We could fork kerneldoc to allow specify relative path of the test which content would be then put into the docs, but that might be a too complicated (+ extra time to contribute to kerneldoc or maintain our fork).

Originally discussed in https://lore.kernel.org/ltp/aDV2pqgZGmKKeJWC@yuki.lan/.
Related ticket: #1243.