Replace doctest with Catch2 in the readme (#421) #403

Job	Run time
Formatting Check	21s
Build and test (windows-latest, openssl_1.1)	8m 2s
Build and test (windows-latest, openssl_3)	8m 4s
Build and test (windows-latest, boringssl)	8m 9s
Build and test (ubuntu-latest, openssl_1.1)	4m 15s
Build and test (ubuntu-latest, openssl_3)	6m 24s
Build and test (ubuntu-latest, boringssl)	8m 8s
Build and test (macos-latest, openssl_1.1)	1m 40s
Build and test (macos-latest, openssl_3)	8m 21s
Build and test (macos-latest, boringssl)	8m 18s
Interop test	1s
Build with clang-tidy	1s
	1h 1m 44s

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