Auto generate documentation for cmake components

[dbarker]

Follow up to #3368 and @owent's idea to generate documentation automatically for cmake components

• Add an optional DESCRIPTION argument to the otel_add_components function
• Generate the documentation block of the opentelemetry-cpp-config.cmake file based on the components and targets installed
• Generate a markdown file with the components and targets supported and link to it with the INSTALL file. (provide instructions on how to generate this file).