Replace doctest with Catch2 in the readme #375

Job	Run time
Formatting Check	21s
Build and test (windows-latest, openssl_1.1)	12m 21s
Build and test (windows-latest, openssl_3)	18m 34s
Build and test (windows-latest, boringssl)	11m 49s
Build and test (ubuntu-latest, openssl_1.1)	4m 24s
Build and test (ubuntu-latest, openssl_3)	6m 19s
Build and test (ubuntu-latest, boringssl)	7m 55s
Build and test (macos-latest, openssl_1.1)	8m 48s
Build and test (macos-latest, openssl_3)	14m 20s
Build and test (macos-latest, boringssl)	21m 44s
Interop test	19m 7s
Build with clang-tidy (openssl_1.1)	11m 32s
Build with clang-tidy (openssl_3)	12m 37s
Build with clang-tidy (boringssl)	15m 28s
	2h 45m 19s

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