8.0 webclient doc xmo

Web client (JS) part of the documentation, probably needs re/proof reading and the rest of the addons/web/doc stuff needs to be either ported or discarded. There's a *Web Client* section with no content in it yet, I don't remember what I wanted to put there. Maybe specific objects of interest (e.g. actionmanager)?

/cc @amigrave @ged-odoo @csnauwaert