Porting example for the documentation

[hodgestar]

The aim is to document the following porting steps:

• Start with a module that uses the old C API
• Switch to an HPy legacy module
• Slowly port methods to HPy one by one
• Switch to a pure HPy module once the port is complete

Todo:

• unit tests of each step (so that the example doesn't silently stop being up to date)
• write documentation of what happens in each step
• some lightweight tooling for keeping the steps up to date (it turns out showing the diffs is not that useful)

The built documentation for this PR can be viewed here: https://docs.hpyproject.org/en/feature-porting-example/porting-example/

[hodgestar]

Ready for review!

[mattip]

I will be hosting a short session next week for an internal company (Quansight) meetup. Is there a chance to get this reviewed so I can use it?

[mattip]

Since it passes tests, maybe we can merge after a cursory review and then fix-up any inaccuracies later

[fangerer]

Is there a chance to get this reviewed so I can use it?

Sure, sorry, I just missed that it is ready for review. Will do the review tomorrow.

[fangerer]

Very nice example, thank you @hodgestar .
As announced, I'll rebase your branch on master and resolve the conflicts (if you don't mind) since it is mostly due to my changes.

[mattip]

Thanks @hodgestar and @fangerer. If there are problems I missed, they can be touched up in subsequent PRs. I think this will really help new users understand more about HPy.