[doc] Restructure the documentation * Create a new index file * Move the sphinx configuration files do the documentation root * Move book/README to dev/documenting.rst * Move book/mode_plan.py to tools/ * Move book/en/images to images * Move book/en/* to book/ * Move changelogs to changes/* * Adapt the Makefile * Add a title to the javascript api index Related to #4832808

Changes
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.. toctree::
    :maxdepth: 1

    3.14
    3.15
    3.16
    3.17
    3.18
    3.19
    3.20