Doxygen format updates, add links to other Golioth docs

[ncmiller]

• Update Doxygen options to match esp-idf SDK
• Fix various Doxygen warnings that were previously uncaught
• Add links to API and other Golioth docs to README.rst

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit c1025b1):

https://golioth-zephyr-sdk-doxygen-dev--pr258-nick-doxygen-ref-b6l4367u.web.app

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