add Keep-a-Changelog CHANGELOG.md; fix API errors and warnings in docs

- CHANGELOG.md: full history back to 0.1.0 in Keep a Changelog format with
  Added/Changed/Fixed/Removed categories and version comparison links; the
  0.8.4 work sits under [Unreleased]. Dates corrected against git tags
  (v0.5.8.1 is 2022-01-13, v0.5.0 is 2021-06-18, v0.4.0 is 2020-12-08).
  release_notes.rst points to it as the canonical changelog; README links it.

- Fix documented APIs that don't exist or are properties, verified against
  the nanobind bindings:
  - set_num_voices() -> num_voices property (faust_processor.rst)
  - PluginProcessor.set_parameter by name -> look up the index via
    get_parameters_description (plugin_processor.rst)
  - AddProcessor set_gain() -> gain_levels property (other_processors.rst,
    user_guide/index.rst)
  - FilterProcessor .freq / set_frequency() -> .frequency property; the
    automation parameter name is "freq" (several pages)
  - createLibContext returns None and destroyLibContext takes no arguments;
    Box operators are arithmetic (boxAdd/boxMul...), composition is
    boxPar/boxSeq (api_reference/index.rst)

- source.cpp: warp_markers docstring said positions are in samples; the
  implementation uses seconds. Un-section the get_automation overload
  docstring, which broke autodoc when nanobind merged the two overloads.

- Fix a wrong Wikipedia link (DAG pointed at Digital audio workstation),
  wrong test-file references, a broken CONTRIBUTING.md link, the removed
  display_version theme option, and the unknown 'faust' Pygments lexer.
  The Sphinx build is now warning-free.