[doc] pep8 and retab in tutorial code example

[doc] correct pip command to have needed dependancies to use the start command I know we want to remove this command but in the meantime having this documentation working seems like something good.

[doc] explain the difference between a relation and an attribute

[doc] plural

[doc] misc, put every reference to something in the code between backquotes

[doc] bold to make reading more obvious

[doc] style, switch to bullet points list to make things easier to read

[doc] explains what the <X.Y.Z> means

[doc] command output style has changed

[doc] always put file name for code example and uses :file:`path` syntax everywhere

[doc] explicitely give the local server url to make this step more obvious

[doc] start the server in debug mode, it's easier during learning

[doc] recommend sqlite during tutorial because it's simplier to use

[doc] add a link to every cube documentation page

[doc] make this step more obvious

[doc] move to latest CW version for cube creation

[doc] missng import for i18n in example

[doc] unused import in example

[doc] output has changed

[doc] blanks spaces to allow example to breath and be more lisible

[doc] default tests template has changed

[doc] default generated filename as changed

[doc] rst syntax bug, blank line needed before item list

[doc] add link to cubes contribution document

[doc] link to current documentation, not old cubicweb.org doc

[doc] translate french to english

[doc] typo, 'CW' doesn't exist in the example

[doc] always indicate the file in which the code should be put

[doc] respect pep8 in code examples

[doc] serie a small improvments

[doc] typo, the field is "name" not "title"

[doc] fix, command "create" is missing base cube