Deprecate qgis_otb_plugin

As proposed on the forum, we would like to archive the project qgis_otb_plugin.

Since QGIS 3.8, this plugin is part of QGIS’s source code. It is not required anymore. People using older versions of QGIS still need this plugin. But those old versions of QGIS are deprecated (QGIS 3.6 went out 2 years ago), so it makes sens to deprecate this plugin too.

Steps

• Update the OTB documentation
• Write a comment in the project's README
• Archive the repository