Internal docs generation updates

[fauxpark]

Description

• Moved doxygen output directory to .build/ so make clean/qmk clean can scrub it
• Exclude atsam stuff
• Add defines for __DOXYGEN__ and PROGMEM - former allows excluding stuff from being documented (LUFA does this)
• Removed -a argument from moxygen command so it doesn't generate this ugly stuff after the headings:
  

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).