@notthatitmatters - FWIW, there is a downloadable User Manual for Sigil. It accessible via a link in the sticky Download thread at the top of the Sigil forum. It's a bit out of date (0.7.?), but I find it adequate for most things.
The most significant thing that's been added in recent times is plugins, and their usage is documented in the Sigil/Plugins sub-forum.
BR
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