[feature] Unified docs building for legacy code

[jsturdy]

Description

Uniform building of docs for legacy code:

• on demand creation of conda environment for python deps and doxyrest (if specified)
  • USE_CONDA and USE_DOXYREST can be set at docs building time to flag these behaviours
  • if USE_CONDA is not set, the environment from which make doc is called needs all dependencies installed
• split source and build directories for sphinx
• use breathe+exhale for default C/C++ docs, with option to use doxyrest
  • also retained previous manual breathe output
• use sphinx-autoapi for python docs automation
  • no need for manually creating the autodoc stubs

Types of changes

• [x] New feature (non-breaking change which adds functionality)

Motivation and Context

Requests for documentation of legacy code

How Has This Been Tested?

https://test-gemdaq-ci.web.cern.ch/test-gemdaq-ci/docs/api/reg_utils/latest/

[jsturdy]

• last changes I think have been pushed.
• docs site updated