Refactor the doc.s.o structure

The actual documentation is divided in 3 sphinx projects :

• docs ( = devel)
• sysadm
• user

For the landing page, I've created a 4th sphinx project, called "general"

Having 4 sphinx projects, linked together by intersphinx, is a source of templates duplication and limits to cross-applications tables of contents (for the landing page in particular)

In order to improve the documentation management, we could merge it in one unique sphinx project.

Before we validate this option, the consequences and repercussions are to be considered, amongst them :

• possibility to make partial builds
• page anchors deduplication (+ update references)

list to be completed...

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Migrated from T4496 (view on Phabricator)