C++ Client: documentation and sample code

[kosak]

This is documentation and sample code for the C++ examples repo.

Notes:

1. I actually don't think this pull request belongs in this repo. I think it belongs in some other cpp-examples repo, which may or may not have been created yet.
2. The website named in the "intersphinx_mapping" clause needs to be updated

I've tested this documentation on Ubuntu 20.04 by doing the following:

$ sudo apt install doxygen flex bison cmake python python3-pip texlive-font-utils texlive-latex-extra texlive-extra-utils graphviz
# sphinx versions >= 4.1.0 don't like my variadic templates
$ pip install breathe sphinx==4.0.3 furo
$ cd cpp-client/doc
$ doxygen && sphinx-build -n . sphinxoutput
$ cd ../../cpp-examples/doc    # this repo
$ sphinx-build -n . sphinxoutput

Then I copy the first dir (cpp-client/doc/sphinxoutput) to my test website http://client.virtual-uk.com/
And I copy the second dir (cpp-examples/doc/sphinxoutput) to my other test website http://examples.virtual-uk.com/

And the second one links back to the first and it looks right.

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[kosak]

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[kosak]

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[kosak]

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[kosak]

OK, awesome. This looks like this works now. Thanks @niloc132.
Attention approvers (or @chipkent) : to make the make the merge happen I had to resolve a merge conflict in sphinx.gradle:

Chip had

pip3 install sphinx==3.5.4 [etc]

and we had it as

pip3 install sphinx==4.0.3 [etc]

so I kept 4.0.3. I'm not sure how much it matters which version we choose, so I just chose the later one.

[kosak]

Chip had

Re the above, nevermind @chipkent , that line was added by @niloc132.

[jcferretti]

Looked exclusively at the .gradle files, nothing more.