Add CMake option to install docs

Description

This addresses https://gitlab.arm.com/networking/ral/-/issues/2. It adds the option to install the documentation alongside the library.

Checklist

• Contribution meets RAL's licence terms
• [] Documentation updated
• "Unreleased" section of the Changelog updated
• [] clang-format and clang-tidy run and changes included (C/C++ code)
• [] flake8 run and changes included (Python code)
• cmake-format run and changes included (CMake code)
• [] Commit message includes information on how to reproduce the issue(s)
• [] Tests added or updated
• [] Tests pass when run with AddressSanitizer

This is a one-line change to the CMakeLists.txt file so there was no need to update the tests or formatting of source files.