Consider mirroring the OpenSAFELY documentation site's structure

[StevenMaude]

As mentioned in #1815, there are differences in link behaviour you can get, resulting in URLs that look fine in the ehrQL preview but are broken on the OpenSAFELY site.

This is because the OpenSAFELY documentation site has paths like /ehrql/how-to/… whereas the ehrQL preview has paths like /how-to/….

This would be more apparent if the preview also used /ehrql for the documentation. What I'm not sure is whether it's possible to do this, and have the main documentation site correctly still keep the documentation as /ehrql (instead of /ehrql/ehrql).