Automatically update `configs.md` in user guide

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
PR #2754 adds configuration options with descriptions and we can generate markdown for inclusion in the user guide but we don't want to make these updates manually.

Describe the solution you'd like
Implement a mechanism to automatically keep the user guide configs.md page up to date as we add configuration options. This could be a script or maybe even build.rs ?

Describe alternatives you've considered
None

Additional context
None