Remove unused file contributors-guide/index.rst

Florent Rougonrequested to merge
frougon/flightgear-documentation:remove-contributors-guide-index.rst into main

As per an earlier comment of mine. This removes the following warning:

.../src/contributors-guide/index.rst: WARNING: document isn't included in any toctree [toc.not_included]

and avoids the need to maintain the list of parts of the Contributor's Guide in two places (this list is already in src/index.rst).

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