Investigate moving to plain Sphinx+Breathe docs workflow

[bryevdv]

Current repo_docs output generates cramped function signatures that are difficult to read:

There is a configuration option that can potentially fix this, but fortunately, the old "public" version of repo_docs does not support it. Newer versions of repo_docs purportedly can generate better output with argument-per-line and syntax highlighting, but newer version of repo_docs are not available to use in public CI workflows. Accordingly, it may be desirable to move to a "plain" Sphinx+Breathe workflow, without repo_docs.

There are some potential issues here as well:

• Stock Breathe may not handle templates/concepts used in the codebase. Investigating possible custom patches used in repo_docs)
• Need to figure out if Breath auto-indexing features can be used to reduce boilerplate .rst file contents