Manual URL requires trailing slash

[mvl22]

https://www.npt.scot/manual

gives an error, because the manual is expected to be loaded at /manual/ and thus the load of the parallel index.md file files.

This is not a problem on the dev site, whose Github hosting acts differently.

It seems trailing slash support is not on by default on the public site hosting:

https://learn.microsoft.com/en-gb/azure/static-web-apps/configuration#trailing-slash

[Robinlovelace]

Let's review after #309 is merged.