docs: add style guide, tidy dev/contrib docs, update docs sites

[wpbonelli]

• add styleguide section and guidelines/conventions to RTD site
  • transcribed from Feb. 2024 developer meeting
  • added very minimal sample module, maybe others can extend?
• clean up DEVELOPER.md and CONTRIBUTING.md
  • describe fprettify and check_format.py usage
  • update outdated parts of contributing guide
  • update outdated TOCs
  • don't detail git commands, contributors can discover these elsewhere
• remove update_fortran_style.py
  • plan to move related efforts to fprettify eventually
  • existing code can be updated manually/opportunistically for now
• minor improvements/cleanup to docs sites
  • rename RTD site "MODFLOW 6 Program Documentation" -> "MODFLOW 6"
  • rename api ref site "Source Code Documentation" -> "API Reference"
  • update api ref site brief to "USGS Modular Hydrologic Model"
  • update section names in TOCs
  • bump copyright date years
  • minor explication

[langevin-usgs]

We're off the ground!

[wpbonelli]

I will plan to merge when the Canonical outage is resolved after discussing with @Manangka who has platform-specific fixes and more to come in a subsequent PR — also may make feedback easier to have an initial render up on RTD