Add optional section dependent on the presence or absence of a mod

[XFactHD]

This PR adds an OptionalSection block tag which allows the contained content to be conditionally rendered depending on the presence or absence of another mod. This is for example useful in cases where a guide describes optional mod compat and wants to hide the relevant paragraph(s) in the absence of the mod and/or hide a description of a fallback mechanism in the presence of the mod.
If preferred I can switch the inversion of the condition from a ! prefix to an additional boolean attribute.

[shartte]

What I am currently pondering about is how to handle this in the site export, and whether we should introduce some sort of expression language, hm. 🤔

[XFactHD]

My instinct for the site export would be to include these sections unconditionally.

[shartte]

Hm, I was thinking they should probably only be included if the mods were present when the site export actually ran. I have to double check the mechanics of this... But if the mods were not present, the content referenced by those sections might also not be present and not be part of the export. 🤔

Yeah I gotta double check that.