Render the feed as a page, not a tag treeUnverified

A browser handed /index.xml shows what the file is - a tag tree, under
a banner explaining that it has no style information - which is the
one page on the site that greets a reader with an apology. It is also
the page a curious reader is most likely to land on by accident, since
every page links to the feed and not every click on that link comes
from a feed reader.

Point the feed at a stylesheet of its own. The transform runs in the
browser, off the XML already in hand, and builds the same masthead,
type and palette as the rest of the site: the feed's address up front
to hand to a reader, then the posts it carries. Nothing a subscriber
sees changes - <?xml-stylesheet?> is a processing instruction feed
readers have always ignored, and the bytes of the feed are otherwise
the same.

XSLT is on its way out: Chrome removes it on 2026-11-17 (chromestatus
4709671889534976), and the tag tree would come back with it. So the
page is written twice - once in XSLT, once in DOM calls - and the
standby is embedded in the feed as an XHTML-namespaced <script>, which
a browser that ran the transform never copies into its output and a
browser that did not runs instead. The two files are meant to be read
side by side, and they emit the same class names, which is what lets
one section of style.scss serve whichever of them ran.

Both are Hugo output formats rather than files in static/, so each
language build gets its own: its own translated labels, its own feed
address, and the fingerprinted stylesheet URL for that build.