Add documentation style guide

As noted here
https://discuss.tryton.org/t/gsod-project-documentation-for-basic-modules/3186/11
a good place for a style guide and suggested documentation structure is in the readthedocs repository.

assigned to @dave

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New review300511003 at https://codereview.tryton.org/300511003/#ps324111002

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I'm wondering if the document guidelines should not be put outside the document on www.tryton.org like we have https://www.tryton.org/develop for development.

Yes, I think this is a good suggestion, so I've updated the review to add the guidelines to www.tryton.org.

review300511003 updated at https://codereview.tryton.org/300511003/#ps308701008

review300511003 updated at https://codereview.tryton.org/300511003/#ps301011002