docs: add readthedocs build using doxygen

[ReenigneArcher]

Description

Adds doxygen docs, with a docs target, and readthedocs configuration.

Todo:

• all builds pass
  • Linux
  • macOS
  • Windows
• add project icon/logo
• fix logo in header.html
• Add regular markdown files to doxygen build
  • convert README.rst to README.md

Screenshot

Preview: https://lizardbyte--71.org.readthedocs.build/projects/libdisplaydevice/71/

Issues Fixed or Closed

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Dependency update (updates to dependencies)
• Documentation update (changes to documentation)
• Repository update (changes to repository files, e.g. .github/...)

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the in code docstring/documentation-blocks for new or existing methods/components

[ReenigneArcher]

See here: https://github.com/LizardByte/Sunshine/blob/459c604803742d0047c4a15ffe5e5649f1c9bdc4/.github/workflows/CI.yml#L1078-L1101

doxygen/doxygen#9694 (comment)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.85%. Comparing base (f1cb666) to head (975a019).
Report is 25 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #71   +/-   ##
=======================================
  Coverage   93.85%   93.85%           
=======================================
  Files          34       34           
  Lines        1839     1839           
  Branches     1023     1023           
=======================================
  Hits         1726     1726           
+ Misses         51       49    -2     
- Partials       62       64    +2     

Flag	Coverage Δ
Linux	89.37% <ø> (ø)
Windows	93.79% <ø> (ø)
macOS	69.38% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/common/file_settings_persistence.cpp	55.00% <ø> (ø)
...n/include/display_device/audio_context_interface.h	100.00% <ø> (ø)
...mon/include/display_device/detail/json_converter.h	100.00% <ø> (ø)
...de/display_device/detail/json_serializer_details.h	100.00% <ø> (ø)
src/common/include/display_device/logging.h	100.00% <ø> (ø)
...common/include/display_device/noop_audio_context.h	100.00% <ø> (ø)
...rc/common/include/display_device/retry_scheduler.h	95.58% <ø> (ø)
...nclude/display_device/settings_manager_interface.h	100.00% <ø> (ø)
...de/display_device/settings_persistence_interface.h	100.00% <ø> (ø)
src/common/include/display_device/types.h	95.83% <ø> (ø)
... and 24 more