Add openssl to the macOS build instructions

[karpovantonme]

Closes #5096

The macOS section installs pkg-config but not OpenSSL, so on a clean machine cargo build stops in openssl-sys before anything is compiled. The Linux section right above already names the native libraries and lists them per distribution, so this says the same thing for macOS and adds openssl to the brew line

Wording of the first sentence is taken from the Linux section so the two read the same way

Three files, because README.md and packages/hurl/README.md are generated: I edited docs/installation.md and regenerated both with bin/docs/build_readme.py github and ... crates, the way bin/docs/update_all.sh calls them

[jcamiel]

Thanks for the PR @karpovantonme

[karpovantonme]

rebased onto master. 2 red windows jobs were libxml v0.3.21 failing its own build script rather than anything here, and your crates bump has landed since, so a fresh run should come back clean

Checks are sitting on approval to start

[karpovantonme]

spoke too soon, the rebased run fails the same way. the line above the crate error is the real one:

C:\vcpkg\scripts\vcpkg-tools.json: error: document schema version 1 is not supported by this version of vcpkg

so the windows runner image has moved past the vcpkg checkout pinned here, and libxml never gets its library. nothing a PR can do about that, and since windows only runs on pull requests, master will not show it either

fine by me if you would rather close this one