[IMP] doc : expand the coding guidelines

- correct typos
- clarify some points, according to the community comments
- precise the xml notation : record tag vs act_window/menuitem/... tags
- add organization of report, wizard, data/demo, ... (directories and naming conventions)
- new conventions for 'symbols'
- ...

Thanks to @rim-odoo for its grammar extremist review, and @tde-banana-odoo for our long debates about wizard conventions.