Add explanation for testing after-commit callbacks

[mmxw]

This section explains the problem with Django's db.transaction.atomic, and why django-subatomic offers a solution to avoid the problem, and two other alternatives considered.

Note: this is added to the main docs page for now, but we can re-arrange it as we go along.

Fixes #35

See following screenshot for rendered docs:

[meshy]

Thanks for this! I think some of the examples might have gone a little astray in translating from our internal docs, so will need some fixing.

[meshy]

Thought: Because each example has slightly different code and tests, perhaps we could put them both side-by-side with https://squidfunk.github.io/mkdocs-material/reference/grids/#using-generic-grids

[mmxw]

Thought: Because each example has slightly different code and tests, perhaps we could put them both side-by-side with https://squidfunk.github.io/mkdocs-material/reference/grids/#using-generic-grids

Grids seem promising - I tried to arrange it using grid but wasn't able to find a way to present the information in an intuitive way, so I re-arranged the code a bit (the screenshot in the description is also updated). Let me know what you think!

[meshy]

Some things to consider before our pairing session:

[meshy]

Thank you!