design-docs: timetable: unify arrival times and margin types

[flomonster]

Idea of new model:

• We could have only one type of margin (like the engineering margin).
• Each margin can have an associated algorithm (linear or mareco).
• This list of margins can be reduced to two sets of margins (linear and mareco)
• This set of margins can be applied using the "AUTO" method avoiding problems of speed continuity.

[flomonster]

We asked the users questions and this modified our base design. https://github.com/osrd-project/osrd-website/pull/162/commits/a0ea66286ceff8012f9e8946695bcd3292daa048

[multun]

I'd love to further simplify the proposal given the user feedback. I can do this next week, or someone can give it a try until then