docs: use 'exec' directive to generate content in examples.rst

[umarcor]

This PR is based on #730, so I'll keep it as a draft until that one is merged.

Currently, we build the documentation by executing tools/build_docs.py. That imports docs_utils.py, where function examples is used for generating docs/examples.rst. All of that is done before executing Sphinx.

In this PR, an alternative solution is proposed. A generic exec directive is added to Sphinx, which allows executing arbitrary Python code somewhere "inside" an .rst source. Hence, docs/examples.rst is created, and the exec directive is used for executing examples() in there. In order to do so, the examples() function was changed for using print instead of writing to a file.

By the way, function get_theme is moved to tools/build_docs.py and tools/build_utils.py is removed.