CONTRIBUTING: Number/renumber the items lists

CONTRIBUTING: Number/renumber the items lists

For some unknown reason, resynchronization at 6) is necessary to avoid
going back 1.

We can currently refer to:
1. to 11. in the 'How to add new code and to update existing code' part
1. to 14. in the 'Code style and generic remarks' part

Remove a trailing space.

(cherry picked from commit f690437568f4735973a3cd72c2cd99c3f592b5e4)

[skip ci]