Improve code annotations and integrate with docs

[mariajgrimaldi]

We should have a way of automatically generating documentation for each filter, just as it happens with openedx-events: https://github.com/openedx/openedx-events/issues/316 Also, in-line docs for each class should be improved. Another thing we should do is to make available the different versions on the docs page

### Tasks
- [ ] https://github.com/openedx/openedx-filters/pull/173
- [ ] Improve in-line documentation for each filter class

[mariajgrimaldi]

I removed the link so this issue doesn't when merging the first task.