Update enchant link in docs

[vkbo]

My apologies if there's a better place to post this.

The link to Enchant on https://novelwriter.io/docs/more_customise.html produces a 404. The current link appears to be https://rrthomas.github.io/enchant/. It appears that they move the repo with the current maintainer, which is less than helpful.

I also suggest that in the preferences dialog, under "Text editing", the tip "Available languages are determined by your system." be replaced with "Additional languages can be added under Tools > Add dictionaries." since that dialog already has most of the information required to add a new dictionary.

Originally posted by @instancezero in #2041