Add an option `doxygen-bin-path` to specify the path to `doxygen`.

[thb-sb]

This commit introduces an option to let the user specify the filepath to the doxygen binary.

This option is called doxygen-bin-path and can be set in the mkdocs.yml under the mkdoxy section.

The following snippet is an example of how to use this new option:

site_name: My Doc
…

plugins:
  - mkdoxy:
    enabled: !ENV [ENABLE_MKDOXY, True]
    doxygen-bin-path: path/to/doxygen/build/bin/doxygen
    projects:
      mkdoxyApi:
        src-dirs: mkdoxy
        …

If not specified, the default value will obviously be doxygen.

[JakubAndrysek]

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