security: Move entire section to Markdown

Retains the existing layout to the best of Markdown's abilities. Only downside is that new advisories have to be manually added to the index page, as it won't be automatically updated anymore.

Advisory details page looks a tiny bit different (old version used a table which can't be easily replicated in our case) but that arguably makes it a bit more readable. Same with the index page - the horrible spacing of the old version is much improved.

Dates are also missing from the index page. There wasn't a straightforward way to do that in .md, and I didn't wanna throw HTML in there :) Let me know if that's needed though.

Disclosure: parts of old .xml files have been thrown into ChatGPT to be converted into the .md layout, as doing that manually for all the issues would take ages. I manually fixed some of the resulting differences, and haven't noticed any broken links or other breakages.

Edit: Layouts have now been slightly adjusted (yay tables!).

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