docs: the instruction of building zenoh-cpp from srouce is missing

[YuanYuYuan]

1. Following the build and install step (https://github.com/eclipse-zenoh/zenoh-cpp?tab=readme-ov-file#how-to-build-and-install-it) can lead to a zenohc not found error since we enable ZENOHCXX_ZENOHC by default.
2. Users may want to build zenoh-cpp with zenoh-c automatically downloaded and installed. For instance, we use the script scripts/install_from_git.sh in CI. It would be nice if we can provide an instruction for this.

[DenisBiryukov91]

Script is really for development purposes and should not be exposed to users. I think we can either mention that we expect zenoh-c to be installed (but the check can be disabled), or say we require either zenoh-c or pico to be installed and instead check that at least one of them is present.

[YuanYuYuan]

Hi @DenisBiryukov91! Yes, we assume the users have zenoh-c (or zenoh-pico) pre-installed. But we provided building zenoh-cpp with zenoh-c altogether previously. Now we lose this convenient option. That's why I suggest adding an instruction for this.

[DenisBiryukov91]

This is intended, since this is how majority of c++ projects are organized: i.e. there is a list of dependencies, that user is supposed to satisfy - automatic installation of the dependencies is generally the work of package managers.

[YuanYuYuan]

I see. Then let's keep the content but fix the building guide in the README.

In Step 2, I think we need to change it to cmake ../zenoh-cpp -DCMAKE_PREFIX_INSTALL=~/.local if we follow Step 1.

As I said we enable ZENOHCXX_ZENOHC by default, Step 2 would cause a zenohc not found error. But I think we don't need it if we only build the zenoh-cpp. Is that correct?