Add documentation for declearn optimizer

[ybouilla]

It has been decided with the team that for the documentation:

• create a sub-section: Optimizers that showcases the use of a declearn optimizer as a tutorial
• in the user guide: explain deeper how declearn optimizers work, and create matrix that shows what optimizer to use for FedProx, Scaffold, FedYogi, ... or other classical aggregation methods
• decide whether to add a warning on existing aggregation tutorial, that such methods will be removed in a future verison of fedbiomed

[ybouilla]

Work started on branch: feature/810-add-documentation-to-declearn-optimizers (PR 818) https://github.com/fedbiomed/fedbiomed/pull/818

[ybouilla]

Switched to to do label, because needs to be updated wrt issues #860 and #861