We should set up redirects for documentation

[roedoejet]

Bug description

https://docs.everyvoice.ca/guides should redirect to https://docs.everyvoice.ca/stable/guides for example.

[joanise]

We already redirect from https://docs.everyvoice.ca/ to https://docs.everyvoice.ca/stable, but the redirect only works at the root, not in subdirectories.

Same thing with https://nrc-ilt.github.io/g2p going to stable, but not in subdirectories.

To redirect all the subdirectories we create, do we need to have a <directory>.html files for each <directory> redirecting to its stable variant? That should work, anyway, I don't know if it's the best solution. Whatever solution we apply here, we might as well also apply to g2p at the same time.