feat: add function and compute task states in user doc

[guilhem-barthes]

Companion PR

• https://github.com/Substra/substra-backend/pull/823
• https://github.com/Substra/orchestrator/pull/366
• https://github.com/Substra/substra-tests/pull/319
• https://github.com/Substra/substra-frontend/pull/297
• https://github.com/Substra/substra/pull/397

Part of FL-1397

[linear[bot]]

FL-1397 Add new statuses on compute task

[RomainGoussault]

thanks @guilhem-barthes!

Could you please add the CP state machine as well?

[guilhem-barthes]

Could you please add the CP state machine as well?

Ok, I will add the previous one and update to the one we discussed in the future batch of PR which update the compute plan flow