Move Doxygen docs to subdirectory

[fauxpark]

Description

• Removed old shell script
• Renamed internals_gpio_control.md to gpio_control.md
• Moved all Doxygen/Moxygen .md files to internals/
• Removed Japanese translations - given the autogenerated nature of these particular docs I don't believe it's feasible to maintain translations for them

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).