docs: Add Conan installation in the documentation

[uilianries]

Greetings, fellow CLI11 community!

We have just added the latest version of cli11 to Conan Center and thought it would be a nice addition to include instructions on how to install CLI11 via Conan in the book folder.

Please let me know if this is okay, or if you would prefer it placed elsewhere.

UPDATE: Some extra information to be more clear about the used command line and links:

I did not mention how to install/configure Conan, because both links to conan.io and conan-center-index has all Conan documentation updated. For instance: https://conan.io/downloads

The latest version of CLI11 available in Conan Center is 2.4.2:

• https://conan.io/center/recipes/cli11?version=2.4.2

or, can be found by running the conan command:

conan search "cli11"

The command conan install --requires="cli11/[*]" --build=missing means:

• conan install: Install a package from a remote. By default, it points to the official Conan Center
• --requires="cli11/[*]": Install the latest version of CLI11 available in Conan Center
• --build=missing: In case not finding a pre-built package compatible, build it from source then.

Regards!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (e4ee3af) to head (6a1903a).
Report is 33 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%            
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  Files           17        17            
  Lines         4546      4583    +37     
  Branches         0       982   +982     
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+ Hits          4546      4583    +37     

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[phlptp]

Looks good to me, but I have never used Conan, so can't speak much to it. I will give a couple days for @henryiii to take a look, then will merge if there are no objections.
Thanks

[uilianries]

@phlptp Thank you for your quick response. Please, feel free to ask anything in case having doubts about using Conan.