IOMixin for DocVec (to/from)

[jupyterjazz]

We want to support IO functionality in DocVec such as from/to json, csv, bytes, etc. For example, to/from json is needed to make it FastAPI-compatible.

However, we can not use the existing IOMixin here as the logic behind DocVec is different from DocList and we'd have to overwrite some functions.

Solution:
Create an abstract mixin containing functions that the two classes share, and for other functions create separate mixins inheriting from the abstract class

[JoanFM]

The description needs to be changed to Doc2Vec

[JoanFM]

When implementing this, please make sure to habdle the case shown in #1490

[JohannesMessner]

I am breaking this up into multiple PRs (will update the list as I go):

• fix: dict method for document view #1559
• #1561
• feat: i/o for DocVec #1562

[samsja]

lets not do too much abstraction here. There are no shared behavior between DocList and DocVec IO. Lets juts implement the function in both, no need for an abstract class until we can provide the same IO capacity for both