Add to and reorder GitHub webhook docs

[davidjb]

This does a small cleanup on the GitHub webhook docs by reordering the instructions in the order which fields are shown to the user on GitHub (reducing jumping around) but also explains the initial "ping" process that GitHub does. It was confusing to see RTD respond with {"detail":"Unhandled webhook event"} to a ping, when in fact this is working-as-expected.

[stsewd]

Thanks!

Can you follow these guidelines https://docs.readthedocs.io/en/stable/docs.html#content?

The rest looks good :)

[davidjb]

Alright, updated in line with the documentation guidelines. There's quite a few parts of that documentation page that aren't in line with the guidelines (eg semantic linefeeds) so that's just been left as is.

I hadn't seen the guidelines for docs so I've opened #5522 to explain they exist. It also updates the install instructions, which are a bit more involved than just to install Sphinx.

[stsewd]

There's quite a few parts of that documentation page that aren't in line with the guidelines (eg semantic linefeeds) so that's just been left as is.

Yeah, we are trying to enforce this on new docs. Old docs are being refactored in small chunks, so they are easy to review. Thanks!